Well, if this is some kind of relationship issue you're having. Why not try talking to him? You know, simply lay your thoughts out there in the open. Obviously don't try to make him feel bad about it, because he probably has his reasons. But just try to come to some sort of understanding.
At the end of the day, you can either (1) 'Allow' him to continue to watch it, whether that means in some kind of moderation or not, or (2) Don't let him watch it at all. Because if it hurts you that much to know he is/does, then you're within reason to express your upset. I'm sure if the relationship as a whole is completely worth it, he'll respect you enough to stop. (3) Watch it with him or just ask him what 'turns him on' and try to spice things up. No harm in getting a general run down on what turns you both on, and it saves you having to watch any of it at all if it's not your thing to want to try. Those are just some suggestions.
It all comes down to considering each other's feelings. There's absolutely no harm in discussing it like two grown adults, but how you want to handle it is up to you. This is just my advice.
