I would argue that the exact opposite is true. Not only for a debate on SSM, but for almost everything else we do from driving too fast as male adolescents to putting on lipstick before going out the door. Children are the reason for it all.
The drive to ensure your genetic material lives on after your cache of it dies is as strong in humans as it is in rats and mosquitoes.
No, the fact that there are gay people proves that not to be entirely true.
The fact that there are heterosexual people who choose not to have children also proves that not to be entirely true.
Some gay people, however, do have or raise children. The decision to legalise SSM will not have any affect on that. If you want gay people not to have or raise children, then that's a separate argument.