In primitive times, before property was invented and we didn't know where babies came from, it probably didn't matter all that much. After the invention of agriculture, we needed some way to determine who got to inherit the farm, or livestock, or other major belongings. Also, some way had to be developed to make sure that a full-functioning family unit was available to take care of the kids, so they were around to take care of you when you were too old to do it for yourself.

Of course, the tribal elders didn't just gather in Council and declare "Lets invent marriage!". It evolved, in a very Darwinian fashion; those cultures that didn't have some sort of culturally enforced marriage ceremony simply were unstable and tended to fall apart. Reproductive and economic management made it essential, and cultures evolve by natural selection just as life-forms do.

Of course, all sorts of religious and moralistic mumbo-jumbo had to be devised to do this, since our biological evolution had already programmed us to make every effort to keep every female in sight pregnant. But this very trait provided the drive to keep the marriage institution going: from the point of view of the individual, if you didn't get in the club, you wouldn't get laid.

We are essentially monogamous by nature, even in cultures where polygamy is allowed it is uncommon, usually reserved as a privilege of the rich. So the marriage contract could be coupled with our biological nature. Our usual preference to have a long-time mate and companion fit in quite well with the cultural arrangements needed to enforce it. This is not to say "primitive" people don't genuinely love their mates and children, evolution programmed us that way as well. Our ancestors were probably no different than we are in that respect. But it sure helps when our social, biological and cultural natures are in synch.

We all know our modern notion of "romantic" sexual love is a relatively recent development. No, people have been falling in love since cave man days, but it didn't really exist as a prerequisite for marriage until very recently. Marriages are TRADITIONALLY family affairs, arranged by parents for their children for purely economic and social reasons. We've substituted all sorts of dodges to obscure this (we tend to select mates from our own race, social and economic class, etc) but the ability to marry across social and economic lines (and now, even gender barriers!) is something new.

This is what drives both same-sex marriage as well as the opposition to it. Regardless of its Paleolithic origins, marriage now has practical benefits and liabilities which affect us today. There are economic benefits to being married even when kids (or even sex!) is not an issue. People simply fail to understand how much their social behavior is driven by ancient cultural forms, how truly arbitrary and artificial all these rules are, and how in so many cases they are simply irrelevant today. And it works both ways. People fear sudden changes or what they perceive as sudden restrictions. We have technical means of controlling our reproduction now, abortion and contraceptives, which to a great extent are replacing the cultural institutions that used to do this. We also have societal means (laws and courts) of managing property and inheritance as well. Even barren or same-sex couples can adopt children and the State feels it has an obligation to intervene to ensure their welfare. Whether this is good or evil, or even practical, in the long term for the human species is debatable. But morality and religion has nothing to do with it.

I'm sure god doesn't give a shit who you marry, and the godly have no business deciding who you should or shouldn't marry, either.