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.