In my opinion, it just depends on how the person in question feels about it. If the person doesn't feel like it's right, then it's best to wait until after marriage, whatever the reason may be. If the person wants to have pre-marital sex, then there's nothing stopping that person. Either gender needs to be aware of the possibility that they might get an STD or terminal illness. If the person is a woman, she needs to be aware that she might get pregnant and that the man may not want to marry her, leaving her as a single mother with child support (unless she decides to get an abortion, if it's legal in her state.) If the person is a man, then he needs to be aware that the woman he sleeps with, may want to continue seeing him, and that if he does, he might get her pregnant, ultimately giving him the choice of marrying her or paying child support.
So all and all, it's not really a religious issue (ie, "God says it's wrong!") It's more of an ethical and health issue that's up to the person making the choice.