Muslim men are allowed to intermarry with any girl that believes in any of the Abrahamic religions, but not allowed with any other religion. Unless of course the girl converts.
The way, intermarriage occurs between a Muslim girl and a non-Muslim guy by first the non-Muslim guy taking an oath to convert to Islam then the actual wedding ceremony between the Muslim girl and the NOW-Muslim guy can happen.
If you're talking about sex, no one is allowed to have sex before marriage anyway.
Stupid either way.