Personally I always assumed sex was marriage, or at least a significant part of it, based off when Jesus tells the Woman of Samaria to call her husband and the woman tells Jesus that she is unmarried and Jesus points out she has been married to 5 different men. (John 4:16-18).
Marriage as we think of it today is an elaborate ceremonial ritual that exists in all cultures and comes in many flavors. In fact the marriage ceremony we know of in the West is actually fairly new, having been mostly developped in the Victorian Era (1800s). If you strip away all the pomp and ceremony you're basically left with two points regarding marriage no matter what culture you examine.
Point 1: Parents give away their children either through arranged marriage or parental consent
Point 2: The man and woman leave their parents care and cleave unto eachother and begin their own family.
Now I am not saying that people should go out having sex before they perform the marriage ceremony or before they are prepared for the responsibilities that come with having a family, but I would assert that if you did have sex with someone before the marriage ceremony you are all ready married to that person and should thus attempt to go about having the formal wedding ceremony. Either way you cut it though, whoever you sleep with, that's who you are married to in my point of view, so stay faithful to that person regardless of whether you went through the Victorian Era Wedding Ceremony or not.
Just my opinion of the matter, and I think either way you cut it, sex before marriage or after, that you should still be loyal to your one woman (or man if you are a woman). I feel as if the marriage ceremony while not being the marriage itself, is at least a good way to strengthen that bond between two mates and also between the couple and God, thus why it would be more proper to wait until after marriage ceremony to have sex. (Plus if you think about it, waiting until after the ceremony is a good test for your prospective mate.)