Q. Where does the Torah forbid a girl to have sex before marriage?
A. Deut. 22:14 expects a woman to come to marriage as a virgin. Deut. 23:18 says, “There shall not be a harlot among the daughters of Israel”. Maimonides holds that this verse forbids pre-marital sex even if the man and woman have had no other sexual partners (Hil’chot Ishut 1:1). To those who object that the Torah allows concubines, Maimonides states that this applies only to kings. Yes, a couple may theoretically bring a marriage into being by cohabitation but this is disapproved of and severely limited by the rabbinic scholars. The ruling principle over many centuries is that if a couple really want each other they should have a recognised, formal marriage ceremony.