Boyfriend watching porn...

Clare
I'm quite okay with anyone watching porn, and if my boyfriend needs a bit of help when I'm not there then that's fine... 
But he's been searching out a few specific porn stars to watch, and that makes me really uncomfortable. I've brought it up to him and he just brushes it off, but he's searching out certain girls, they look a certain way and do something that I mustn't do for him, that hurts... 
Any tips on how to get over this, or what to tell him? I really need advice :( 
(Ps this isn't because I don't want to do certain things or have sex often, I have a higher sex drive than him and am very open minded.)