I have a serious problem with gay marriage, as marriage is a religious ceremony, so the state should stay out of it. Civil union is the state sponsored joining, and should be the proper avenue for the state to allow something that religion indicates is wrong.
The problem is, the state can't recognize marraige without defining it. If you agree that the religious ceremony has no legal significance (that is, married couples also need to get a civil union if they want the state to respect their union), then fine; but if you want your marriage ot mean anything to the state itself, the state can't avoid deciding what it considers a valid marriage - and then carrying the moral and legal responsibility for that decision, if it would happen to put citizens into different categories based on religious beliefs. Indeed, it would be forced to recognize an official religion that gets to choose.
So, the only way to get the state out of marriage is to go pure civil union route and ignore whatever religious or other ceremony anyone feels fit to add on their own time.