While the Bible proscribes quite a lot of sexual behavior, I can’t recall any specific passage that says that men can’t marry and have sex with prepubescent girls. I don’t doubt that the tradition was that girls should at least have reached menarche before marrying, but any written rule in the OT about it?
Here’s a thread on this very issue.
The Bible is not a textbook, nor a guidance manual, nor an encyclopedia of allowed and forbidden practices. Whatever else you may believe about the Bible, it’s not organized that way. The fact that some particular act isn’t mentioned in the Bible doesn’t tell you much.
The Rabbis, IIRC, have established that the age for marriage is 12 years, 6 months. But this was concluded extrascripturally.
The prohibitions on specific sexual “abominations” in the Book of Leviticus are mostly related with how the heathen nations around the Israelites performed and even gave official sanction to some of them, and the instructions were for Israelites to avoid joining in. But apparently the compilers figured there was no risk of the Israelites taking up sex with six-year-olds.