"So how is stopping two people of the same sex from being married any different from stopping a man and his adult daughter from being married? On what grounds would you allow one over the other?"
Because government sanctioned marriage is an endorsement and an incentive. We don't endorse most things just because they are allowed -it two different questions. I'm allowed to over eat, be lazy and not go to church but I don't want the government encouraging those things either.
So, in other words, you just want the ones that YOU think are OK to be allowed. Same sex, OK... Father-daughter, no. You can't seem to define the difference, just that you think it's OK.
Actually, at one point you advocated same sex marriage, then later you said no. I guess I really don't understand where you stand.