What is not publicly known is same-sex relationships have higher "divorce" rates, higher incidents of domestic violence, both on each other, and any children in the home. Its the only situation where CPS does not automatically remove the child by default because they can't risk a counter charge of discrimination. Suicide is higher, substance abuse is higher, mental & physical health issues are all more complicated. Children raised in the situation have an even tougher time. All of this is documented medically, legally, etc. Were it any other "group" it would be treated differently.
I have been thinking about what 45-70 said though, and I think it may have merit. I've always felt marriage is a divine institution, to be conducted accordingly. I'm not convinced that a legal divorce necessarily registers in heaven anyways since last I checked, God is not subordinate to our laws. And the Constitution is notably silent on the issue. Things with legal status but no other value live longer than they ought to. Remove the legal status, and a form of social darwinism kicks in. Without the legal protection, society doesn't have to worry about stigmatizing or discrimination - you can point at something wrong and call it wrong. Regardless of your faith, most folks look at same-sex, multiple-partner, pedophilia, beastiality, and without much help, reach the conclusion that it's not right. Take away the legal recognition and we can't teach that crap in schools, put up with it on tv, or accept it in mainstream. We can prosecute predators of all kinds. As long as the government is involved in marriage, they can change the rules whenever they want, which only works when the wolves and sheep agree.