Summary
Genesis 2:23-24 and Matthew 19:4-6 make it clear what God holds to be natural in human sexuality: that sexual relations are to be limited to a man and a woman who are married to each other. From the outset, this inherently rules out all other “possibilities”.
Content
The basic idea
Genesis 2:23-24 and Matthew 19:4-6 make it very clear what God holds to be natural in terms of human sexual behavior. There is exactly zero wiggle room here.
It is almost amusing how clear the text is. If people would just read this passage in Matthew with an open mind, it inherently rejects all sexual deviance in that it clearly states that God’s intention is that a husband and his wife be joined together as one flesh.
In other words, even if we had absolutely no other passages forbidding various forms of sexual sin, Matthew 19:4-6 alone would be sufficient, in that it clearly states what God’s intent is with respect to human sexuality: that it is to be limited to a man and a woman who are married to each other. All other permutations (man and man, woman and woman, man and woman that are not married) are off-limits.
It is “wife” not just “woman” in these passages
I suppose I should note that in Genesis 2:25 and Matthew 19:5 both, the word translated as “wife” also means woman generally. The word is ishsha (Hebrew: אִשָּׁה) in Genesis, and gune (Greek: γυνή) in Matthew.
In both languages, the normal word for woman also means wife. (The same holds true for man/husband—אִישׁ in Hebrew, and ἀνήρ in Greek). To a degree, this shows just how fundamental the concept of marriage is—that linguistically, being a man and being a woman is implicitly connotatively tied to being married. (At least in these two specific languages).
For other usages of these words where “wife” rather than “woman” is definitely in view, you might compare Proverbs 5:15-19; 1 Corinthians 7:2; Ephesians 5:33.
I am bringing all this up mostly since I made such a big deal about how self-evident Matthew 19:4-6 is. If someone were to try to wriggle out of marriage being a requirement on the basis that the word might simply mean any old woman, well technically that could be possible lexically.
But other passages in the Bible make it plenty clear which way we must translate all these passages (i.e., as “wife” rather than just “woman”). For example, Hebrews 13:4.
So I suppose it would more precise to say Matthew 19:4-6 makes everything crystal clear when taken alongside Hebrews 13:4.