Marriage licenses, at the very least, require explanation. It would be interesting to know what other countries do on this issue.
One argument on this issue might be this. People who enter marriages naturally bring children into the mix. But having children means adding mouths to feed, and children to educate, etc. So the government compensates people who risk this with various financial advantages. However gay relationships don't incur this risk, so they should not receive this kind of compensation.
One firm conviction I have on this issue is that my government is not competent to adjudicate the moral question of homosexuality one way or the other. I am against sodomy laws and forcing bakers to bake rainbow shaped cakes with two grooms on top for exactly the same reason.