Based on a comparison of various translations of this passage, it would appear that the conjunction γὰρ does not necessarily always imply a direct dependency. The NETBible, for example, translates it as "in fact".
So Jesus appears to be using this conjunction to "pivot" from the Sadducees misunderstanding of marriage in the afterlife, to their misunderstanding of resurrection in general. In other words, He is not saying that there is no marriage because there is no death; rather He is saying that there is no marriage AND there is no death in the resurrection because it is a totally different type of life. So there is no contradiction with the idea that the institution of marriage preceded the entrance of death and sin. He is merely pointing out that ALL of these things will not exist in Heaven.
Regardless of how the particle γὰρ is translated, I don't think a causal relationship between marriage and death can be avoided in these verses. If we take γὰρ in its basic sense of "for", then Jesus seems to be saying that there will be no marriage in the resurrection because there will be no death. To understand why Jesus would say this, we must first understand the question he is answering.
The Question of the Sadducees
The Sadducees question is based on the law of brother-in-law marriage found in Deuteronomy 25. This law was instituted because people die. Thus, if there is no death, this law becomes unnecessary. The Sadducees thought the implications of the law made the resurrection absurd, when in fact the implications of the resurrection annul the law.
NT Wright, in his book The Resurrection of the Son of God, writes:
A key point, often unnoticed, is that the Sadducees’ question is not about the mutual affection and companionship of husband and wife, but about how to fulfil the command to have a child, that is, how in the future life the family line will be kept going. This is presumably based on the belief, going back to Genesis 1:28, that the main purpose of marriage was to be fruitful and multiply...Because in the new world that the creator god will make there will be no death,...hence [there will be] no need for procreation.
Neither the evangelists, nor Jesus, nor his interlocutors, face the question which occurs to us: if marriage is designed to procreate the species in the face of death, why does Gen. 2 describe it being instituted before the fall? The only answer seems to be that the present question and answer remain limited by the implied scope of the Levirate law.
If we understand the limited scope of the Sadducees question and Jesus response, then we can perhaps reconcile what Jesus says about the future of marriage and what he says about its institution before the fall.
Some possible answers may be found in 1 Corinthians 7. Here, Paul explains the necessity of marriage for those believers who are oppressed & tormented by instinctual (natural) sexual lust.
But about the Resurrection to the Deathless state, Paul states:
29. What I mean, my friends, is this: there is not much time left, and from now on married people should live as though they were not married; 30 those who weep, as though they were not sad; those who laugh, as though they were not happy; those who buy, as though they did not own what they bought; 31 those who deal in material goods, as though they were not fully occupied with them. For this world, as it is now, will not last much longer.
Luke 20:36 is about the Spirit where as Matthew 19 & Mark 10 are about the worldly laws of morality.
In Luke 20:36, Jesus appears to connect the absence of marriage in the resurrection with deathlessness (as shown by the use of the Greek term "gar"), suggesting that marriage and death go together.
But in Matthew 19 and Mark 10, He seems to teach that marriage existed before the Fall (and thus presumably before human death). How can these two passages be adequately reconciled?
When God created Adam and Eve, after they were married, He commanded them “to be fruitful, and multiply, and fill the earth.”1 We may assume three things: (1) marriage exists for procreation, (2) procreation exists to fill the destitute earth, and (3) the destitute earth would eventually become full at which point procreation, and thus marriage, would no longer be necessary.
In the world to come, after the resurrection, the kingdom of God (new earth?) will be full of those who are predestined to live therein. Since they will no longer die, being immortal, marriage will no longer be necessary since procreation will no longer be necessary since the kingdom will always be full. This seems to be to reconcile the supposed difficulty.
1 Gen. 1:27–28, 2:24 cf. Matt. 19:3–6
Did Jesus teach that marriage existed before the Fall?
In Matthew 19, Jesus uses the Jewish significance of scripture to answer a question about divorce in a way that challenges the Jewish significance of scripture.
He answered, “Have you not read that he who created them from the beginning made them male and female, and said, ‘Therefore a man shall leave his father and his mother and hold fast to his wife, and the two shall become one flesh’? So they are no longer two but one flesh. What therefore God has joined together, let not man separate.” Matthew 19: 4-6
Jesus referencing scripture is not necessarily Jesus verifying that what is written/read is truth - only that it is written/read. The Sermon on the Mount is a perfect example of this.
What Jesus teaches from this particular reference to Genesis is not that marriage existed before the Fall, but that, once married, a man and his wife 'are no longer two but one flesh.' When the law of Moses allowing divorce is brought up, Jesus dismisses this law as a concession to the Jewish people's 'hardness of heart', and reiterates his teaching:
"And I say to you: whoever divorces his wife, except for sexual immorality, and marries another, commits adultery.” Matthew 19:9
Does Jesus suggest that marriage and death go together?
Jesus is responding in Luke 20 to 'the Sadducees, those who deny that there is a resurrection', who are trying to show the incompatibility of scripture with the resurrection by their hypothetical wife of seven brothers. The reason for the law of Moses referenced by the Sadducees is revealed in the original scripture:
If brothers are living together and one of them dies without a son, his widow must not marry outside the family. Her husband’s brother shall take her and marry her and fulfill the duty of a brother-in-law to her. The first son she bears shall carry on the name of the dead brother so that his name will not be blotted out from Israel.
However, if a man does not want to marry his brother’s wife, she shall go to the elders at the town gate and say, “My husband’s brother refuses to carry on his brother’s name in Israel. He will not fulfill the duty of a brother-in-law to me.” Deuteronomy 25: 5-7
Marriage in both Moses' and Jesus' time was deemed a solution to the threat of non-existence in death. It provided a way for men to ensure that children born to a particular woman continued both their genetic code and their name - evidence of their existence - after their death. So marriage is a way of avoiding this sense of non-existence that comes with death, by ensuring one's name (self) lives on.
The necessity of carrying on a name through children within a marriage is only required when one faces death as the end of one's existence. But this is not the case in the resurrection:
And Jesus said to them, “The sons (and daughters) of this age marry and are given in marriage, but those who are considered worthy to attain to that age and to the resurrection from the dead neither marry nor are given in marriage, for they cannot die anymore, because they are equal to angels and are sons (and daughters) of God, being sons (and daughters) of the resurrection." Luke 20: 34-36
If they cannot die anymore, then they need not strive to replicate themselves in order to carry on their name. Therefore, they need not marry nor have children, because their name continues through living to God. Jesus then went on to demonstrate how Moses understood that Abraham, Isaac and Jacob all continue to 'live to God' in this way, and their name is carried on:
But that the dead are raised, even Moses showed, in the passage about the bush, where he calls the Lord the God of Abraham and the God of Isaac and the God of Jacob. Now he is not God of the dead, but of the living, for all live to him.” Luke 20: 37-38
So, is it better not to marry?
This is also linked to the following passage in Matthew, when the disciples suggest that 'it is better not to marry':
The disciples said to him, “If such is the case of a man with his wife, it is better not to marry.” 11 But he said to them, “Not everyone can receive this saying, but only those to whom it is given. 12 For there are eunuchs who have been so from birth, and there are eunuchs who have been made eunuchs by men, and there are eunuchs who have made themselves eunuchs for the sake of the kingdom of heaven. Let the one who is able to receive this receive it.” Matthew 19: 3-12
Jesus suggests here that there are those who choose not to marry nor have children 'for the sake of the kingdom of heaven'. I would imagine that Jesus counts himself as one of these, as would Paul and many later saints. The kingdom of heaven is then built through their example: showing that they need not have offspring or marry in order to carry on their name. Like Abraham and Moses and Jesus, their name continues in the way they live to God.
How does one live eternally by living to God?
The way it's described, this way of carrying on one's existence sounds a lot like fame. But building monuments, inscribing one's name on everything, getting one's name up in lights or their face on TV - these are not eternal.
Elvis 'lives' for his fans - not through his fame, his music or his movies, but through the continuing impact on their own lives of the unique way he connected with the world. Jesus lives for believers in this same way - not through the written word or documented proof, but because the way that his unique expression of God's love and connection impacted on the lives of writers in the early church continues to impact on the lives of believers today.
Jesus' name was never documented during his life, nor was it carried on through marriage or offspring - in this way, he experienced not only death, but the apparent non-existence that comes from death without his name being carried on. His name was essentially 'blotted out from Israel'.
Yet the name of Jesus lives on over two thousand years later in the way that he connected with others: his examples of unconditional love, fellowship, non-judgement and inclusion, in what he shared of his relationship with God, and his invitation for others to connect with God in the same way, via his living example.
We married initially because we feared that death brought about non-existence: we are driven to replicate ourselves through offspring guaranteed to be ours, and carry on our name (an extension of ourselves) beyond our death.
The resurrection obviates the need for marriage and children for those who are considered worthy, by ensuring one's name, their existence, continues beyond death as they 'live to God': ie. express God's love and connection through the unique way that they connect with the world.
TL;DR: it is not "marriage and death" which go together, but "marriage and finite individual lifetimes on earth that are orders of magnitude shorter than mankind's total lifetime on earth, i.e. from creation of man to the end of times".
Let us demonstrate the above going step by step.
The primary end  of marriage is procreation.
Before the fall, people needed food for sustaining their lives just as we do now. This is clear from the Gospels' narrative showing Jesus, whose human nature enjoyed the same preternatural gifts as Adam's and Eve's before the fall, feeling hunger (Mk 11:12).
From 2, the quantity of people that can live concurrently on earth at any time, with or without a fall, is bounded by the quantity of food that is available to them.
Since the earth is finite, the quantity of food that can be produced globally in a given time, with or without a fall, is finite.
From 3 and 4, the quantity of people that can live concurrently on earth at any time, with or without a fall, is finite.
If, in a no-fall scenario, people stay on earth indefinitely and the new generations keep getting married and procreating, the quantity of people living concurrently on earth will tend to infinite, which contradicts 5.
Therefore, in a no-fall scenario:
either people do not stay on earth indefinitely,
or else, after the total number of people living on earth reaches a maximum value M determined by the maximum quantity of food that can be produced globally, everyone stops procreating. In this case, the total number of people that will reach the Beatific Vision at the end of times will be just M. Also, since the last generation will have to abstain from procreating, if the population grows geometrically by a factor of K with each successive generation (as can be expected in a no-fall scenario), then a fraction of (K-1) / K of all people that ever existed will have to abstain from procreating. Thus, if population doubles with each generation, 1/2 of all people that ever existed will have to abstain from procreating.
Therefore, if God wills either that the number of people that will reach the Beatific Vision should be greater than the number of people that can live concurrently on earth, or that substantially more than 1/2 of all people that ever existed should have the possibility to procreate, or both ends, then people should not stay on earth indefinitely in a no-fall scenario.
If we assume (IMV quite reasonably) that this was the "actual in the sense of designed though counterfactual in the sense of achieved" no-fall scenario, then it is not "marriage and death" which go together, but "marriage and finite individual lifetimes on earth that are orders of magnitude shorter than mankind's total lifetime on earth, i.e. from creation of man to the end of times". This does not imply death or even aging. Rather, people could reach their optimal biological state of development, e.g. at an age of 35 years, and remain in that state until they are e.g. 144 years old, when their body would be glorified by God and they would be taken to Heaven without going through death .
 With "end" understood in a teleological, not temporal sense.
 The Apostle Paul mentions this case twice: when referring to the people still living on earth at the time of the Parousia and final resurrection (1 Cor 15:51-52), and when stating his wish that this were the actual case for him (2 Cor 5:4).
In Luke 20:36 Jesus states one of the reasons for not marrying and being given in marriage is because the righteous resurrected can’t die anymore but will be like the angels in heaven. The contrast to angels is not about sexual desire, he's contrasting mortality with immortality. God creating sexual relationships before humans sinned and could die contradicts death being a reason for sex. Only legal customs of marriage were invented after humanity sinned, so that's all Jesus could be referring to. The original Greek in the account with the sadducees for marry and given in marriage meant the male proposer paying a fee to the woman's dad and the woman being given to the man respectively, because she didn’t get a choice. This was not done with the first human couple so says nothing about marriage itself. Would any of the people who heard Jesus answer been amazed at it if he meant sexual feelings and relationships would be eliminated? I’m pretty sure Jews had as much sexual desire as most other people, so would have felt despair if that was the context.
At the beginning God said for us to be fruitful, multiply, renew earth and subdue it with no indication it was to ever stop. Being alone was the one thing God said during the original creation that is not good, made Eve to complete humanity with both genders and said that is the reason a person will leave their parents and be united with a spouse to become one flesh. Marriage was needed to eliminate loneliness, not friendship. Reproduction and representing Gods ideal relationship with people are never stated to be reasons for marriage. God can't change his mind about what's not good if he is the same forever and celibacy is not something deemed good during the original creation. After God created marriage he declared everything very good because there was no longer anything missing required to make anything better. Most Christians have claimed that something unspecified by the bible would be required to make life better forever and also make marriage and sex useless, ironically ignoring the fact that marriage was required for creation to not be missing anything good. It's contradictory and more akin to Buddhism than Christianity. If having something better is a reason to eliminate sex, it's a reason to eliminate everything God made, and we should all just feel Gods presence, getting joy from that and never being with anyone else or doing anything else for eternity.
If simply being more aware of God's presence and love makes marriage unnecessary, then God would have had no reason to create gender and sex to begin with, because God and Adam had exactly that kind of relationship people will have with him in the renewed creation, before Eve was made. If simply being part of God's image is all the reason needed to have gender, then the external features that distinguish the genders faces and body shapes to attract each other sexually, and the hormones and parts used for sexual pleasure, reproduction and for babies would all be wasted. That’s all part of what makes us the genders we are and God doesn’t create anything to not be used. The human female figure is shaped the way it is to fit babies during birth. keeping a libido is justified. Gender having other functions is no reason to get rid of any of them, or any function except one isn't needed and there's no more reason to have other functions.
If people interpret Jesus answer to the sadducees as claiming no one will be married anymore, then we can't be married to Jesus after the last resurrection either. The description is given of God as a husband to his people in parts of the old testament, like Hosea, Jeremiah and Ezekiel. That wasn't meant to replace sexually intimate marriage anymore than being described as Jesus bride is in revelation. They are different types of relationships that fulfill different desires.
What most Christians have claimed is a delusional interpretation claiming that loving something more than anything else God made but not caring about it anyway because of what the saved will experience is not a manipulation of free will. There being no disappointments isn't teaching that the restored creation will automatically be satisfying even if he eliminates what produced the most pleasure of it. God created specific things to fulfill specific desires. If there is no more marriage for sexual intimacy, then the most passionate desire for anything God created will be the only exception to his creations being restored, but he teaches he will restore everything. There's a whole book devoted to the joy of sexual passion, Song of Songs. There isn't a bible book entirely about the joy of any other creation, or celibacy. Heaven isn’t the final destination, it’s the renewed earth. The only thing that needs elimination is what sin did, because there was nothing wrong with anything God made.