What do YOU want to do. If you don't want to do it then its a no, plain and simple. Don't ever do something you really don't want to do in order to please someone. Its your RIGHT. If he doesn't like it then he can get lost.
But remember if he didn't want to do something he wouldn't do it. And theres never any guarantees that he'll be around forever. Would you regret giving him anal if you broke up someday? Theres your answer. Stick to your guns!