Of course I can.
Marriage is about subsidizing the creation of the next generation of mankind. Used to be, if you couldn't get pregnant together, you could get an annulment, and the marriage was considered dissolved.
My position, and, in a democratic society which runs on consensus, I don't actually need to defend it, is that if you get married, and enjoy the privileged status that that entails at my expense, you OWE me several well adjusted children to carry society forward when I retire, and if you get divorced, you OWE me for all the benefits you enjoyed at my expense. If you are infertile, you can't get married. If you are too old to have children, you can't get married. Live with who you want, fuck who you want, but marriage is about families, and if you're not interested in making and raising one, then leave it to others.
Religion really doesn't have anything to do with it. Homosexual marriage is an outrageously, ridiculously irrational thing all by itself.