Need help about my boyfriend's unrealistic sex.

My boyfriend and I have been together for 7 years now. About half of our relationship has been long distance because I went to college and then took a job 6 hours away from home for one year. So here's my issue, throughout the years we've had phone sex, sexting and did stuff on the Web cam to satisfy Ourselves. Watching pporn together was something we'd do too, sometimes. Now, lately he's been wanting to talk dirty about me fucking another guy while watching. (He wants to do it in real life). I told him over and over again that I don't feel comfortable talking about it and all I really want to do is talk about me fucking him and only him. Although he says he'll keep the details to himself when the conversation comes up, he DOESN'T FAIL AND TALKS ABOUT THE SAME THING WHEN HE'S HORNY. I'm tired of it because I don't want to talk dirty to him about me fucking someone else! !! And now I ignore my messages when he wants to talk dirty because I don't want to hear it anymore. I don't know how to handle the situation. I don't understand how he gets turned on seeing me fuck someone in his head. I've asked him why and he says he just wants to pleasure me. I tell him if I wanted pleasure from someone else I'll go do it, but I obviously don't want that. What do l do? Is this normal for a boyfriend to fantasize about their girlfriend having sex with some other guy???