Not all those examples were between a man and a woman. There are several examples of a man and several women, or a woman and several men. Some ancient cultures held the love between men as the most pure.
Also, marriage is, today, a legal construct. As such, it is within the authority od the legislators to redefine it.
As for the human race vanishing, you'll have to realize that preference is not learned. The gays don't learn to be gay, it's genetic, just like I couldn't be taught to be gay, as I am a genetic heterosexual. So, it won't affect the numbers either way.
But, bottom line, for me, is that I can't let my preferences dictate the lives of others. I'm pretty much a "people can swing their fists around as long as they stop before it reaches my nose"-guy. What other people do in the privacy of their home or how they define their relationship does not hit my nose.
I can also speak from experience. Sweden has allowed same sex marriage for a long time now, and was one of the first countries to do so, and it hasn't have any negative impact on society so far. If anything, it has increased tolerance and reduced discrimination and attacks on gays.
Also, I don't know how much you know about Swedish politics, but you might recognize my name. That's because my sister, up until a couple of months ago, was the first openly homosexual party leader of a major Swedish political party. So, let's just say that I have some insight into the issues involved.