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We see in Leviticus 15:18 where the man and woman are unclean until evening after a man lies with a woman and there is an emission of semen. Why does this make them unclean?
[Lev 15:8 KJV] 8 And if he that hath the issue spit upon him that is clean; then he shall wash his clothes, and bathe [himself] in water, and be unclean until the even.
Your question is well focused: "Why does this make them unclean?".
Taking aside the Lucian comment ("For obvious physical reasons") we must think about several actions an Israelite may perform in a normal day. Some of them - linked with a job, or with a physical/biological necessity - make him dirty. For an example, to defecate is a normal (and necessary) action we perform (we hope) in a day. Since the natural cycle of intake-outtake nutrients/refuses was created by God we don't find in the Bible that IEUE (יהוה) considered unclean a person who performed this action. Granted, God expected that he/she wash the dirty body part of him/her, but, in every case, him/her was not considered unclean "until the evening", like in other instances.
Someone may object, now, that also the copulation (inside the marriage) is a God-created natural action. This is true. But, in this case, a big difference intervenes.
The apostle Paul in Galatians 3:19 wrote: "What, then, was the purpose of the Law? It was added in order to show what wrongdoing is, [...]" (Good News Bible).
Paul himself explained that sin was transmitted through physical generation (conception): "What follows? This comparison. Through one man sin entered into the world, and through sin death, and so death passed [διηλθεν, an aoristical form of the Greek verb διερχομαι, 'to pass through'] to all mankind in turn, in that all sinned" (Rom 5:12, Weymouth).
So, differently from the original declared God's purpose (Gen 1:28), man - after his sin - was able to transmit not an eternal life to his sons, but only sin, along with a very limited life (compared to the endless life).
Now, since the Law was issued "in order to show what wrongdoing is" (like we have read in Gal 3:19) it's only obvious and consistent, from IEUE's part to enhanced that copulation included (in addition to the procreating and pleasure factors) always the 'passing-through' of sin. This did help the married couple to think over this important topic and encourage them to not lose sight the truth that their life depended not only on familiar or physical/biological needs (Mat 4:4) but on the search of a friendship with God (Isa 41:8), taking into an account - in our life - the absolute badness of sin, so that striving to avoid it was an action approved, blessed by God.
You seem to be mixing up verses about (general) discharge and emission of semen.
The discharge in Lev 15:1-15 is not semen. Note in Lev 15:7, it states that it is a man with discharge, so pus from an infected wound would qualify, but so would blood and other body fluids. Lev 14 specifies that such uncleanliness will last at least 8 days.
The verbiage regarding semen is quite different. In Lev 15:16, it is not a "discharge" but "seed of copulation" that "goes out of him". The uncleanliness from such an emission lasts at most a day (till evening).
Note also that the menstrual flow of Lev 15:19 is also referred to as a "discharge".
Biblical cleanliness is ritual (spiritual), not physical. It is not about pardoning offenses or sin. It is about being presentable to Yahweh (Deu 23:14). No rationale is necessary aside from that it is commanded, or seen favorably, by Yahweh.
While disease is considered ritually unclean (much of Lev is about disease states), that which is ritually unclean is not necessarily physically unclean, and not all physical uncleanliness is ritually unclean.
The verses in question illustrate the disconnect between physical and spiritual cleanliness. Even though they have bathed, they still remain unclean until evening (Lev 15:18).
Jesus explicitly states that physical uncleanliness does not make a person spiritually unclean. (Mark 7)
A modern example is the use of Holy Water, which may contain numerous culturable critters that have been deposited by hundreds of people. To not use the physically contaminated Holy Water is to risk ritual uncleanliness, but to use it is to become physically unclean, while becoming ritually clean.
Another example is the pasteurization of non-kosher foods. Pasteurization makes the food physically safe for consumption. However, it remains ritually unclean (not kosher), even though it has been made physically clean.
The LORD YHWH provided no qualification as to "Why does this make a husband and wife temporarily unclean?". Only that it is so.
The whole text is found in Lev 15: 16 And if any man's seed of copulation go out from him, then he shall wash all his flesh in water, and be unclean until the even.
17 And every garment, and every skin, whereon is the seed of copulation, shall be washed with water, and be unclean until the even.
18 The woman also with whom man shall lie with seed of copulation, they shall both bathe themselves in water, and be unclean until the even.
In all issues (pun) regarding uncleanness, YHWH lists this reason plainly, which we can apply to this in context of verse 15:
31 Thus shall ye separate the children of Israel from their uncleanness; that they die not in their uncleanness, when they defile my tabernacle that is among them.
32 This is the law of him that hath an issue, and of him whose seed goeth from him, and is defiled therewith;
33 And of her that is sick of her flowers, and of him that hath an issue, of the man, and of the woman, and of him that lieth with her that is unclean.
From a Christian or post- manifestation perspective, the whole weight of the mater falls upon how we understand the Tabernacle or Temple which is imposed entirely upon the worshiper by such verses as 1Co 3:16 Know ye not that ye are the temple of God, and that the Spirit of God dwelleth in you? 17 If any man defile the temple of God, him shall God destroy; for the temple of God is holy, which temple ye are.
As well as the collective body of Christ in: Eph 2:21 In whom all the building fitly framed together groweth unto an holy temple in the Lord:
This is not a new concept which came with the Apostles doctrine, however, as it is drawn from
Lev 26:12 And I will walk among you, and will be your God, and ye shall be my people.
Thus showing a harmony in the continuity of God's desire with or without a Temple settings. Therefore, it is absolute that the Christian Israel of God should be holy and separate according to 2Cor 6:16 ...for ye are the temple of the living God; as God hath said, I will dwell in them, and walk in them; and I will be their God, and they shall be my people. 17 Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing; and I will receive you, 18 And will be a Father unto you, and ye shall be my sons and daughters, saith the Lord Almighty. Repeated in Revelation 18:4.
Hereby, it is made abundantly clear that Christians as the Israel of God and as his temple are called to separating and observing the Laws of Holiness which are only taught in the Torah / Law of YHWH GOD.
