Another christian POV which is probably heretical to other groups-

I think the marriage/sex thing is an error in order and that the social construct was created to demonstrate the importance of sexual relations. It's a case of God stepping in because we're a pack of screw-ups and need a lot of guidance.

We tend to confuse sex and love so "just sex" causes strife. In at least some sense, you are mated to anyone you share a sexual relationship with and will carry less emotional baggage the less times you sever those ties. I personally feel that many couples are emotionally married before they decide to have a wedding. Not only does sex = emotional ties but sex = babies (& future of the species). On the most practical level, babies need people who must take care of them; if everyone knows it's your kid you gotta take care of it. We all know breakups are hell esecially for those babies, so preventing all but the unavoidable ones is important: both prohibiting doomed marriages and solving problems inside marriages. Finally, if you create a family unit it should be recognized and protected--in the way-back-when, a man "took a wife" by public proclamation, not by ceremony and paperwork. Because we accept cohabiting couples, we have many "married" people without legal protections they would have had under common-law rules.

But like I said, we all mess up so I'm going to congratulate anyone who really belives in forever on their 3rd marriage.