Several posters, (e.g. Giraffe, LHOD and Matt_Mcl) seem to be confusing the issue here. Of course it’s very easy to say “gays yes, others no”. No great revelation there - sorry guys. The issue is about the justification for doing so.
Whether woman could or could not think is not a social reality - it is a fact that is either true or not true. People’s attitudes about women might be a social reality but while that had an impact on whether they could vote, voting itself is just an act. The only thing in the way of allowing this act was people’s opinions that it was a bad idea - to the extent that you disagree with the decision you would have to allow it.
The thing about marriage is that it is purely a sociological institution. The fact of married people being considered one family unit is in the minds of people - people consider the bonds of marriage to be cohesive to the point of making them one unit, to an extent. (The same could also be said of children - while children are a biological fact, the idea that this conveys some special relationship between children and their parents is a sociological one.)
My position is that the government should not be in the business of trying to dictate to society which bonds are to be recognized and which are not. The government should not try to reshape society in any way, other than to stop people from harming each other.
If people are opposed to gay marriage because they think gays are scum that don’t deserve the advantages that come with marriage, that should not be a factor in governmental recognition. But if people oppose it because they don’t consider this particular bond to be a binding one of the nature of marriage (or parenthood) that should be a factor.