Right! The teaching of all major religions going back thousands of years, all the marriage laws and jurisprudence of all previous governments and societies, and the personal beliefs of 2000 years of previous generations... all, all of, must now be worthless.
Apparently to some all wisdom begins the day one is born. Most certainly on this issue because to hold the belief that:
1) marriage requires a bride and a groom
2) a husband and a wife compliment each other in a way two members of the same gender cannot
3) or that children benefit from having both a mother and a father
I'm so glad you mentioned this. I've been thinking about this discussion today, particularly Anitram's very succinct definition of the institution she will soon be partaking of (Congrats, Anitram, by the way!).
I think what you are missing is that the definition of marriage has actually changed quite a bit over the millenia. Indeed there is very little that hasn't changed. Let's consider the criteria under Canadaian law that Anitram described.
1. Mutual consent. This was not always a given in marriage, and even today in cultures where arranged marriage is still the norm, consent while not ignored is certainly not the predominant factor.
2. Exclusively between two people. Clearly, as has been discussed, marriages have not always been limited to a mother and father but often a father and many mothers.
3. Conjugal relationship. Sex has always been part of the marriage equation (though how much of it actually happens throughout the marriage obviously varies!), but what hasn't always been part of the marriage relationship is the assumption or the need for a romantic attachment as a precondition to marriage or as a necessity for the success of the marriage.
Now considering your traditional criteria above. The second two have not been a given among all societies and all cultures for millenia. The whole idea that a child must have a mother and a father or that the man and woman complement each other are not universally preached ideas that have only now been challenged. If anything, the tradition has been that of the village or at least the extended family being crucial to the raising of children.
The only thing that really hasn't changed over the millenia is your first criteria in that marriage must always involve at least one man and one or more women. The reason for that is simple. For most history marriage has been essentially about one thing: The production of children. And without a man and one or more women, until recently it was not possible to produce children.
You see the real change in marriage is not, as you posit, the change from one man and one woman to two adults of either gender, but from marriage primarily as a vehicle for producing and rearing children to a marriage primarily as a vehicle for a lifetime of intimacy and commitment with another person. That changed happened with heterosexual marriages and then it was only a matter of time before homosexual relationships would also enter the picture.
Finally, the wisdom of history is over-rated. For generations, slavery was considered acceptable in most cultures and societies. For generations, war has been considered a legitimate way to resolve disputes between societies. The weight of history does not afford either of these traditions any special authority. Past generations were not any wiser or any dumber than we are today.
