Why are gay marriages outlawed?

A lot of the arguement that most are not getting is that the same couples aren't getting insurance and tax benifits of the said union, it has NOTHING to do with they want to say they was married before god (ok, most aren't worried a ton about it). I sadly am very confused because I agree with both sides. I agree that a union of 2 people should receive said benifits reguardless of BUT I agree with most religions that marriage is for man and a woman. I personally can't see why we can't have a "domestic union" for anything that doesn't fit into the marriage role. What does anyone care that the (just to use as a example) Cathotic church doesn't think you are married, seperation of church and state,,,,,,,give them the benifits (and the problems of divorce) of a union.