Procreation should be not just a right, but a basic good afforded to all human beings. One can voluntarily give up the right to procreate, one can give up that good. But homosexuality claims that procreation isn't a basic good at all- and forces the partners in a homosexual relationship to give up that right and deny that good. This is inherently unjust to homosexuals to claim that once they have a gay relationship, they can never have a straight one, and it is wrong for homosexuals to force the person they claim to love to never procreate.
This isn't important to teenagers, but it is important as people grow older. In my extended family, I know at least four heterosexual couples who have gotten divorced over infertility issues. I expect the same, now that same sex marriage is legal in Oregon, will happen to homosexual couples at a much higher rate.