As i ponder on it, i tend to lean more towards the theory that societal rules and religion was created by man to prevent him from doing wrong. Here is an example.
I would say that live-in relationships are fine, totally. After all, we, as evolved people seek loyalty, faith from the other person. If it exists in a live-in relationship, why not? To the question of one not being faithful, well, it can happen irrespective of whether one is married or not. That's really not the point. The crux lies in one being true to another. A marriage stresses or imposes (for want of a better word) this on the man and wife, and hence 'sanctifies' the bond between them.