Why do guys with girlfriends watch porn?

(I'm sorry that it's long, please bear with me!) I feel like my boyfriend watches porn too much. I just don't understand why? Please don't say "well you look at men too." Because the truth is, everyone once in a while I'll see a guy and think "oh he's attractive," but it's not even on a weekly basis. I don't watch porn or look at other guys naked or think about other guys naked. I think my boyfriend is the sexiest thing ever and he's all I need! Then I also see guys saying "porn doesn't demand us to do things" or "it takes less effort to jerk off to porn than to have sex." First, I'm not demanding of my man at all, unlike "most" females. If he does something for me, it's completely of his own will and I do things for him as well. Second, that just sounds like you're being lazy and selfish and you'd rather have something other than your wife/girlfriend. I'm willing to have sex with my boyfriend whenever he wants it, yet I'm STILL catching him watching porn. It really bothers me too because all he watches is lesbians and has talked about threesomes a lot in the past, but I can not bring myself to do that because it feels like cheating and is completely against my morals. :( I also feel like it's totally cheating if he's having sex with another girl, nonetheless in front of me?!? I honestly feel ugly and unwanted when my boyfriend watches porn. I keep hearing "well most guys do it," but why? It's fake and completely unrealistic. Real women don't look or act like that. If that's what really turns you on, what do us women have left? It's impossible to please you. You always just want something else. I feel like watching porn is the equivalent of receiving nudes from a bunch of "sexy" friends/females and jerking off to those or visiting those friends/females and jerking off in front of them. Sure you're not touching them, but you are still getting sexually aroused and pleased by THEM. To me it's all cheating. :/ I feel like sex should be something intimate between you and your partner, not something you do with everyone. It really upsets me, I've cried many nights over this fact. I just want to know WHY so many guys do this to us? It really hurts. I just want to be loved and appreciated. That's all I ask. I'm a simple woman & easy to please. (By the way, I am in no way religious. I do have values and morals though.)