I think the issue here is really about accessibility. If it was porn stars in a video you'd have the same reaction? nah, you'd probably just look the other way and let him look at his videos. But with the escorts they are advertising that actual experience, and it's only a quick dial away.
You disapprove of the fact that she can be contacted. if he really wants it he can have it, and maybe that's what he gets off on -the idea that the girl is close. Porn is voyeurism, maybe he just likes the extra rush that you get when that person doesn't feel quite so far away and removed.
But as for the real reason why, only he really knows and what he is telling you is that it's no big deal. Either you'll trust him or you won't, and that's a whole other issue.
This is a clear example of projection. The men replying to this question and finding a way to blame her for "bitching" and "not letting have is fun" clearly didn't read her question very well.
To me it seems Brook is completely fine to his masturbating to porn (it's pretty well known these days that men do this, no big deal).
Her main concern is why is he so adamant (WITHOUT HER ASKING) about hating hookers so much and then going back to it time and again.
You guys have to read the question before going off in your nagging trips on the poor girl who asked an innocent question about her boyfriend's obvious inability to work out a problem, and not any concern about the actual masturbation.
I can say sexist things, too: "Men... they just don't listen to the facts and just make up stuff in their heads turning a situation into a different problem from what it actually is."