Thank you. How to understand "virgin," and then "pure virgin," is a series of knotty theological issues that I should try and help disentangle from ordinary use of these terms
Paul's basic advice seems to be "avoid idolatry," but the logic of seeing a Christian (who might well be a mature man) as a "pure virgin" is rather involved.
In Jewish law, having sex doesn't create impurity. The issue here is menstruation. The key concept behind "pure virgin" might be the girl needs to do the ritual clean-up (mikveh) before she's ready for sex.
Paul's logic might be that the Christian baptism is a "spiritual" mikveh, preparing the Bride of Christ for her encounter with the deity. To go into temples and get ritually dirty is a problem the new husband won't like. It makes ritually clean sex impossible.
I don't know if this applies to people when there's no temples around. It may be that the destruction of the Jewish temple marked an ending point for this discussion. But in any event, any Christian girl or boy is "married" to Jesus and their touching isn't a ritual violation.
I've just spent another chunk of time reading about the "slandered bride" of Deut 22:13-22, but it's something of a riddle. Premarital sex, which isn't a crime in OT law, seems inadequate to explain the death penalty offense.
I suspect the problem there is the new husband isn't sure if a conceived child is his own. It's not male vanity—control of the genealogy is a central theological objective. But there's many views.