A more difficult passage to some, yet saying the same thing with Jesus's same intent is found in Matt. 19:3-12 —
3 The Pharisees also came unto him, tempting him, and saying unto him,
Is it lawful for a man to put away his wife for every cause? 4 And he
answered and said unto them, Have ye not read , that he which made
them at the beginning made them male and female, 5 And said , For this
cause shall a man leave father and mother, and shall cleave to his
wife: and they twain shall be one flesh? 6 Wherefore they are no more
twain, but one flesh. What therefore God hath joined together , let
not man put asunder. 7 They say unto him, Why did Moses then command
to give a writing of divorcement, and to put her away ? 8 He saith
unto them , Moses because of the hardness of your hearts suffered you
to put away your wives: but from the beginning it was not so. 9 And I
say unto you , Whosoever shall put away his wife, except it be for
fornication, and shall marry another, committeth adultery : and whoso
marrieth her which is put away doth commit adultery. 10 His disciples
say unto him, If the case of the man be so with his wife, it is not good
to marry. 11 But he said unto them, All men cannot receive this
saying, save they to whom it is given. 12 For there are some eunuchs,
which were so born from their mother's womb: and there are some
eunuchs, which were made eunuchs of men: and there be eunuchs, which
have made themselves eunuchs for the kingdom of heaven's sake. He that
is able to receive it, let him receive it.
Here Jesus is confronted with a legal question by the Pharisees, who were attempting to trip Him up. Their context was Deut. 24:1-3, —
1 When a man hath taken a wife, and married her, and it come to pass
that she find no favour in his eyes, because he hath found some
uncleanness in her: then let him write her a bill of divorcement, and
give it in her hand, and send her out of his house. 2 And when she is
departed out of his house, she may go and be another man's wife. 3 And
if the latter husband hate her, and write her a bill of divorcement,
and giveth it in her hand, and sendeth her out of his house; or if the
latter husband die , which took her to be his wife;
4 Her former husband, which sent her away , may not take her again to
be his wife, after that she is defiled ; for that is abomination
before the LORD: and thou shalt not cause the land to sin , which the
LORD thy God giveth thee for an inheritance.
It was not lawful for a man to take back an original wife whom he had divorced,after she married another, and her second husband died.
Here you see Jesus state God's original intent*,"...the twain shall be 1 flesh."* This is the One Covenant still in existence before the downfall, that 'God' from Heaven sees 1 body. Therefore, whatever a husband and wife do together(in love) is covered under the 1 flesh covenant. Paul illustrates this in 1 Cor. 7; and in Heb. 13:4, —
Marriage is honourable in all, and the bed undefiled: but whoremongers
and adulterers God will judge.
So, to understand Jesus's command, one must understand God's Original Intention, which existed before the Fall, and the One Flesh Covenant He makes with a man and woman rightly joined together in marriage. Another discussion is marriage entered under duress, false pretenses, etc., these are not "God Joined" marriages-therefore do not come under the "One Flesh" Covenant.
What is interesting is what Jesus tells His disciples(in private) after His confrontation with the Pharisees in Matt. 19.
First, when confronted by them about "Can a man put away his wife for any cause(no fault divorce)", and "Why therefore did Moses allow a Bill of Divorcement", Jesus responds, "Because of the hardness of your hearts." First of all, Moses never gave them 'carte blanche' to divorce their wives, it was for "uncleaness" (adultery, fornication) yet it could be interpreted 'disfavor', which an "evil and adulterous generation" could construe as meaning 'for whatever reason'.
So when He says,"But I say to you....", He has rolled back that understanding; instead, re-establishing 'uncleaness' as meaning fornication or adultery (fornication would be sexual union w/one who is unmarried, adultery is with one who is). One cannot be 'joined' to one who is joined to someone else unlawfully, this is clearly His intent.
Now, after He reproves the Pharisees, telling them that "...whosoever divorces his wife, or marries a woman who was put away (for adultery) commits adultery", His disciples have a huge question, and it goes like this: 'What about the one who has been unfaithful to, it would be better off for them not to get married in the 1st place. Are they to remain 'eunuchs' (those who are incapable or taken a vow of celebacy)?
Then Jesus responds,"To those who are given this 'command' by God, let them 'receive' it, meaning unless you are given direction by God(and the accompanying grace) to remain single, then get married(in the Lord). This does not apply to the offender, only the offendee. The 'offendee' (if through no cause of their own, God judges the heart) is divorced by their spouse, then they are "free from the guilt of adultery, and may take another spouse(of course, in the Lord)". The offender is charged with additional sin, which is to "drive their partner in the arms of another man/woman" which is tantamount to adultery.