Why does my boyfriend stare at other girls?

[R]

My boyfriend looks at other girls and it bothers me and I get really jealous and insecure. I mean he doesn't break his neck but I see him look and he's always looking at girls pictures on facebook but never adds them. It bothers me so much. I know I can't blind fold him and he's going to look wether I like it or not. I've told him it bothers me and he says he doesn't look at them but I know he does and he tells me he does when we argue. How can I not get mad at this even when I see him look? Please help.