Jealousy.

I don't entirely understand women who get jealous when their boyfriends look at porn. I understand strip clubs being more of a problem and hookers more still, but outside the health/ethics issues with a hooker these things really never bother me. They're not competition. A pornstar will never take him home and a stripper will never go hiking with him. They might get his cock, but they're no threat to his heart.

Unless he gets obsessed with them, but then your problem is less "that thieving slut" and more "that psychotic man."