The thing is- the government IS involved in marriage at the current point in time. By saying that gays cannot marry, which denies the lgbt community hundreds of rights that married couples have, the government is involved in it at the present time. I think there needs to be a true separation of church and state.
Even though I think Obama could do more for gay rights, like I said, I would pick Ron Paul as my second, but only because he would probably leave things as they are at the moment. I think I saw an article on him saying that he would leave it up to the states to decide, saying that it's not a federal issue. All the other Republican candidates at the moment seem as if they could damage to gay marriage, especially Rick Santorum.