Oh.. and some of my said 'friends' reasoned that if we permit same-sex marriages, more and more people will be gay, and that the world population will decrease because guys will not have children.

Ironically, if this were actually true, it wouldn't be a bad thing considering how overpopulated the world is.
Also, legalizing same-sex marriage would likely be a symbolic step of acceptance towards homosexuality that would promote more people to admit to being gay, or for bisexual persons to choose a same-sex partner, so it could lead to more openingly gay people. I would not be surprised if gay couples had fewer biological children than straight couples. Assisted reproduction is very expensive and I doubt any couple could afford to do it more than once or twice. The question would be if these persons would have less biological children as part of a gay couple than they would if they were closeted in a hetrosexual relationship...That'd be an interesting study, but I digress...Either way the statistical difference would be tiny compared to the world population since the US contributes so little to its growth currently.
Try that on your 'reasoning' friends.