Sexuality itself is a complex issue that will continue to evolve over and over. To me, allowing same-sex marriage is not as complicated. It doesn't affect my marriage if Sue and Joan down the street get hitched. I think that making same-sex marriage laws about sexuality as a whole is spreading the issue and overcomplicating it.
However, I also feel that population growth itself is not important. In fact, I have made the personal decision not to reproduce; I would rather, if I ever do decide to have children, adopt or foster. While not passing judgement on those who do choose to have children, I would feel socially irresponsible adding more to our already-huge population.
I am not alone. I'm too lazy to grab the sources, but it's been shown that the more educated women are, the fewer children they decide to have, and that holds true for women in Africa, in Afghanistan, in the US, across all sorts of cultures. You can probably Google the source if you want more info.
On the other hand, some of the most stable family units I've known have been led by same-sex parents. I grew up with a girl who had two moms. She was the biological daughter of one mother (who got a gay male friend to donate sperm) and the adopted daughter of the other mother. She's one of the most well-adjusted people I know. On the other hand, it was unusual in my schools - all of them, and I went to several - for children of hetero couples to have both parents still at home. Divorces and separations are pretty common. So I think that the focus on one man, one woman, the focus on a mom and a dad to be the center of the family unit, has become somewhat outdated.
Those are my views, from my personal experience.