I have been asked, 'where does it say you can't have sex before marriage in the bible?' Proverbs 5:20 is my answer; this verse reads, "do not be intimate with a strange woman, neither cling to the embrace of a woman not your own."
Strange woman is anyone who is not your wife. When it says "woman not your own," that, to me means your wife. This verse can have an effect for women too. If you are not a woman who belongs to a man (your husband) he doesn't belong to you and therefore you should not sleep with him. If you are not married practice abstinence. If you are married, only sleep with your spouse.