I've been following this thread though I live in Canada where gay marriage is already legally acknowledged, but I'm trying to understand your logic. While I don't believe that condoning gay marriage will start a process that would eventually lead to no heterosexuals, lets just say for argument's sake for a moment that it would.
If the entire world was to consist of only gay humans I seriously doubt that would be the end of our species anyway. Why? We are logical creatures by nature. We understand the basic concept that we must procreate in order for our species to survive. Many gay people feel a very strong desire to procreate; they just don't want to do it the "traditional" way. As a race, we would still have babies but would find other ways to achieve this. It may be more challenging for 2 men to have children biologically related to them, but it's pretty easy for 2 women. You could argue that this is not natural though - it's not how nature intended it to work. So what? If it was against the laws of nature, then it shouldn't work, but it does. If you still want to argue that it's not natural so it's still wrong, how do you justify medicine? Medicine is mankind using logic and knowledge to interfere with a natural process. Someone who has diabetes or any one of many other diseases would die if it wasn't for humans medically interfering with the disease's natural process. Should we stop all medical interference with nature because to do so is unnatural? If someone gets sick or hurt, should we wait and see if nature heals or kills them? Should heterosexual couples who cannot conceive in the traditional way not be given any medical assistance to help them procreate because it would be unnatural?
I'm not posing these questions in a snide manner, I'm trying to understand how you define "natural" and know where to draw the line.
Even in the unlikely event that the human species turns all or mostly gay, we would still survive.