When I was in college, my sociology textbook included a discussion of marriage, and mentioned that an effort in one state to make marriage legally become a three-year contract with options for renewal "was treated as a joke by the other legislators". When I read that I couldn't help but think, It was a joke. I mean, suppose that someone said that he thought that football would be a much better game if instead of using an oblong ball, we used a spherical ball; and instead of kicking or carrying the ball, we hit it with a heavy stick; and instead of playing on a rectangular field, we played on a diamond-shaped field; etc. We could certainly debate whether or not such changes would make a better game, but surely if we made such drastic changes, what we would have would no longer be football. It would be a lot more like baseball.

Marriage has historically been understood to be a permanent union between a man and a woman. To the best of my knowledge, every culture in the history of the world up to 1990 has understood this to be the definition of marriage. Even in those few cultures that have viewed homosexuality positively, I cannot find a single example of a culture where a homosexual couple would be considered "married".

Yes, there is variation in the marriage laws and customs of different cultures. Some have allowed a man to have more than one wife; a few have allowed a woman to have more than one husband. Some allow marriage of an adult to a child or between two children. A few have prohibited or discouraged marriage between people of different social classes or races. There have been different rules for for when divorce is permitted. (Other than bigamy, which has been widely accepted in many places at many times, all the other variations have usually been controversial wherever practiced.)

That's about it for the variety. I don't know of any culture that has ever viewed marriage as a three-year contract with options for renewal. I don't know of any culture where it was viewed as normal for a married couple to not live together. I don't know of any culture that allowed a person to marry himself. And I don't know of any culture has ever allowed a man to marry any sort of creature or being other than a woman: No culture has ever allowed a man to marry a goat or a box of rocks.