I believe that civil marriage, asside for offering an equal stand in front of society as for straight couples, also offers certain rights.
I am not sure what's the deal with your medical system in US, ours in Romania is very different, but there is something about gay partners not being able to cover one for the other certain medical costs with the insurance. And I guess that is the case with the spouses from a straight couple marriage.
In my country, if they would get the right to get married, they could, for example:
- legally adopt children
- legally obtain a loan based on both incomes (impossible if you are not married, in here)
- legally leave something for their life partner as heir / heiress, if they pass away. You can even if you ar not married, but it's very hairy, usually relatives of 1st , 2nd degree jump in, get a lawyer, claim the deceased was crazy and the money / whatever it is in the will should be theirs, etc.
I can't think right now of other economic / social marriage benefits. I'm sure there's more after all, I am happily married LOL
But anyways, the idea is that if we allow gay people to get married, their life quality would improve. And even ours, indirectly. More families, more incomes, more loans, more stability, more kids adopted....less drama...