You don't need to go to a church to get married. You can get a full blown marriage in front of a judge at McDonalds if thats what floats your boat.
So since gays can get MARRIED in front of a JUDGE, without any religeous mumbo jumbo in their ceremony at all...
Why do you care?
As for why someone would want a marriage instead of a civil union (even if civil unions had everything a marriage does, which it doesn't) is probably the exact same reason why a black person wants to be able to drink out of the same water fountain as a white person does. It doesn't matter if the "black" water fountain is identical to the "white" one; people don't enjoy being treated as lesser humans because of racial/social/sexual differances. Collectivism sucks, and is for sucky people who like sucking.