Well, I have never really understood why anyone would watch porn or whatever they do when masturbating together - it's a very personal thing, even when you're in a relationship. I mean, you can masturbate with your partner or share fantasies for foreplay etc., but somet things just feel private. But it's weird that he's not honest about it, have you told him you have no problems with him masturbating or watching porn? Maybe he believes you will get angry.

From what I deduce from the little you have told us, he seems to kind of be turned on by sexual promiscuity/deviance, or taboos if you like. Prostitutes=promiscious and a taboo. Granny, tranny and bbw (at least some of it)=taboo, and sometimes promiscious. Or risque, if you like that word better. In other words, he gets off because it's a bit "forbidden" and very sexual. Much like how kids are obsessed with sex when it's "forbidden" (which is also why preaching abstinence will never work). I think it is completely harmless, but maybe you should try suggesting (or simply trying) some roleplaying, for example? Something to spice up the sex life a bit, make it a bit more taboo/risque.