I wouldn't do something that I was morally against.
The question of whether or not to allow gay marriage is mainly a question of things like whether gay people should be allowed to visit their loved ones in the hospital.
To prevent them from doing so is very morally wrong.
I wouldn't want the work, and I was about to tell you that I wouldn't go in, but maybe if you went in and purposefully did something to not get the work, then you would have wasted the time of the people trying to advertise against it and you wouldn't have to participate in it, and your agent wouldn't be mad at you?