I'm a happily married woman and I masturbate to videos of people having sex (including naked males who are not my husband) and I also fantasize about all kinds of other people and situations. It's normal, though not everyone feels the same way about it.
I used to feel the same way that you do, when I was just starting to learn about my own sexuality and what goes on in the minds of both men and women. That was several years ago... and it takes some time to let go of the need to be jealous or insecure. But I assure you that it is completely normal for people of both sexes to look at all kinds of images/videos when they masturbate, and that it has nothing to do with you.
However, if you are anything like the way I was, nothing that we tell you is really going to make you feel better about the situation. You are going to have to work through these feelings on your own, and by talking about them with your boyfriend. You are both going through a time of exploration and learning about each other, and your own self. Give it time... and keep talking with him, not working against him. Explore yourself... do you like to masturbate? You could talk with him about that, and maybe see how it feels to watch porn with him, and on your own. Just try some things and see how it feels.
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And think not you can direct the course of Love;
for Love, if it finds you worthy, directs your course.
--Khalil Gibran
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