Thinking of others besides gf when i masturbate

Okay, so there's something that's really f**ked up my relationship lately. I love my girlfriend to death and i really think she's special, and could be the one i want to grow old with. But one day she says something while we're driving around that leads me to tell her that i think of other people, not just her, when i masturbate, most of them being her close friends. Now of course i would never cheat on her and i never really wanted to do any of these things with these other people, but i still thought of them and would picture me and them doing sexual things together. One of these people I really dont find attractive or like at all and I even thought of her. And I had never felt guilty about any of this stuff before we talked about it because I guess I never really thought about it as being wrong. I also used to watch porn during our relationship, and never really thought about it being wrong, but I told her and she flipped, so i stopped because i cared about her feelings. I just want to know if any of this is normal behavior?

Thanks.