When my boyfriend and I are watching a movie at home (which generally means that we are going to have sex), if a sex scene comes up that is longer than, say, a minute, and that is quite graphic, it immediately kills his mood and he doesn't want to have sex anymore. He becomes uncomfortable and embarrassed for some reason. If we were playing with each other during the scene, he loses his erection and even after the scene is over he isn't able to get back to where we left off. Needless to say, it's quite frustrating as I am looking forward to having our usual amazing sex, but just like that his mood is ruined. I am also a bit worried because he feels bad about it and I don't want him to feel guilty, also I don't want him to think of sex as something that he "owes" me even when he's not in the mood, or anything like that.
I asked him why this happens (in the most calm and empathetic way possible) and he wasn't able to give me an answer, all he said was that watching long-ish sex scenes (not to mention porn) with me makes him uncomfortable and it kills his mood for the rest of the night. He asked me why it didn't have the same effect on me, I told him that while it does bother me to do stuff with him during those scenes (I find them distracting and mood-killing), once they're over I'm able to get back to us.
So my question is, do you have any theories as to why this happens? Does this happen (or has this ever happened) to you and/or your partner, and if so what is/was the reason? I'm just looking for hypothesis' here, I know that the only one who can truly answer this question is him. Thank you in advance :-).