Boyfriend looking at other girls' facebook photos.

My boyfriend of four years is constantly (bi-weekly) stalking other girls' fb photos. It's the same girls every time, a couple of his co-workers, one of my friends, and a girl he used to have a thing for. Every time I type Facebook into the search bar, I see their names or links to their photos in the suggested lists. He is 29 and I am 26. We've lived together for three years and other than this (and some bad kitchen habits) I'd say our relationship is great. It really hurts me, I don't mind porn at all but the fact that these are women in his life is what gets to me. Does he not see his cute coworkers enough? Does he need to look at the same 12 sexy photos of them over and over? Some he's not even Facebook friends with. I know I'd have relationship problems with anyone but this really hurts. I have not confronted him about the issue, and I'm sure bottling it up isn't helpful. I feel we're getting too old for this sort of thing.