You've forgotten what we were talking about. You suggested that religion was the reason why marriage has historically been heterosexual. I supplied some non-religious avenues of explanation, which transcend religious differences. Whether or not they are good reasons for the heterosexuality of marriage is not the point. The point is that you are wrong to single out religion as the only possible answer.
No. That's not what I said.
What I said is that religion is the only reason to fight it now. I said nothing about reasons for the supposed historical state of things, which, as DeSelby pointed out, is not what you think, anyway. I also said that "many people did it this way for a long time" is not a good reason for much of anything.
You're also comparing ideas that have drifted in and out of fashion with certain groups of people, with an idea that has always been believed by the vast majority of people.
And that is not correct either. I am not talking about small groups. Flat Earth thing was pretty damn universal for a long time. Miscegenation was widely enough feared that it made it into law in more than a few places. Owning slaves was also widely acceptable to the vast majority of people for many centuries and across cultures, yet it is deemed wrong now.
Again, we're discussing the origin of mankind’s firm conviction that marriage requires a male-female dynamic. It is beyond obvious that it probably arose from the fact that families were united and new families created by procreation (that's biological and involved sex, you know). Since homosexual sex doesn't create children, it's easy to see why marriage only made sense for male-female couples.
Nether does sex between people beyond a certain age, sex between people with certain disabilities, sex between people who choose not to reproduce. And some people can't have sex at all. All of those can marry, and by and large always could. What makes them different from a homosexual couple? Sex is something that may happen, but is not part of marriage as a legal construct.