Just to let mm know how different state laws can be, in Montana all I have to do to be married is say that I am. Assuming I have a girlfriend, of course. The two of us merely need to say "We're married" and we are, without a license, ceremony or anything else. She can start calling herself Mrs. ParkingPlaces and we can start living happily ever after post haste.
Except, if I were involved, things wouldn't work out. But for others, it works fine.
Of course, being a red state full of gun nuts and Obama haters, gays cannot do this, or hook up legally in any other way either. Yet.
The current common definition of a marriage is not all that relevant, because we are trying to change it. If person A and person B love each other, and want to live together via a commitment, their sexual orientation need not be an issue. Hey, we're all supposed to love each other, right? Why get picky about such things?
Only to produce kids? Like someone else said, we don't put anyone in this country through a fertility test, so for now, that certainly can't be the prime reason folks marry. Granted, in many cases kids are produced, either on purpose or accidentally, but that is as much a side effect as it is a purpose, in many cases.
And many fertile and infertile couples (and individuals) adopt. A pretty nice thing to do. A little kid needs a parent or two, people volunteer, and whammo, a family. So even if fertility were the biggest concern, I would hope that the law would allow for infertile couples to adopt as an alternative. Including, of course, gay couples.
One of the more vocal anti-gay guys I know, who apparently goes out of his way to tell everyone it is immoral, spent a couple of days in the hospital three years ago because he got beat up by the husband of his girlfriend. But that's different I guess. Some christians seem to think it is okay to choose which morals they want to adhere to.
An interesting interpretation I once heard about the "man not lying with another man as he would a woman" involved the biblical inferiority of women, and claimed that what the bible was saying is that a man should not treat another man as an inferior, as he would a woman. I like that. It remains highly offensive, and though it wrongly disses women yet again, at least it doesn't create a whole new hated subgroup, and it gives me hope that I can find a new interpretation of the shellfish as an abomination thing, so I can eat clam chowder again.
Oh wait, I'm an atheist, and can do that whenever I want. Never mind.