Anxiety he's thinking about other girls?

Does anyone else get anxiety when you're having sex or giving your bf a blow job and he closes his eyes for a while that he's thinking about other girls or porn stars? It sucks. That thought always runs through my mind. One time he "jokingly" asked if he could fuck me and think about a porn star while doing it, and I'm just afraid he actually does that. There's no way to know and it's driving me a little insane. When we have sex he doesn't care to take my top off so he doesn't even look at my body unless we're doing doggy which we don't often do. And my tits aren't huge so I can only imagine he's thinking about a girl with a huge rack or something.