I think this young man (and perhaps the young woman) may have taken some things heard at church that were intended to discourage pre-maritial sex, heard half the message & twisted it to suit him. I seem to recall being told something like, "Having sex binds you together like husband & wife. Save it for marriage. You're not ready (too young) for marriage. Don't have sex." He/they only heard the first sentence & ignored the rest.

Probably using as supporting passage Genesis 2:
24: Therefore a man leaves his father and his mother and cleaves to his wife, and they become one flesh.

This passage is quoted by both Jesus when speaking of divorce & by Paul in couple of places for example

I Corinthians 6:
15: Do you not know that your bodies are members of Christ? Shall I therefore take the members of Christ and make them members of a prostitute? Never!
16: Do you not know that he who joins himself to a prostitute becomes one body with her? For, as it is written, "The two shall become one flesh."
17: But he who is united to the Lord becomes one spirit with him.

The thinking here is, now that we've become "one flesh" we must be married.

There's also the marriage by rape, or how do get Dad you let you marry him when Dad said no passage in Deuteronomy 22:
28: "If a man meets a virgin who is not betrothed, and seizes her and lies with her, and they are found,
29: then the man who lay with her shall give to the father of the young woman fifty shekels of silver, and she shall be his wife, because he has violated her; he may not put her away all his days."

Here are some bible links: RSV & KJV or Bible Gateway