Sex before marriage is a sin, but...?

Ok, so I know that the bible states that sex before marriage is a sin. I asked one of my friends why this is, and she said that if you have sex with someone who isn't your husband, even if it was before you were married, it's said that you committed adultery. However, what if you had sex with the man that you ended up marrying before you got married, would that still be considered sin? I mean, I realize that the chances of a guy sticking around after the sex is slim to none, but theoretically, would that be acceptable?