I agree with the above.
Also -- when a guy wants to watch porn enough that it causes disagreement in a couple, that's actually kind of a peculiar area. Many men sometimes watch a little porn, for example when on a business trip and alone at the hotel, and wives shrug it off, figuring boys will be boys -- they prefer real sex with her, so what the heck. But in this forum we do often see women writing in that have been fighting with their boyfriend about him watching porn, making him promise not to, catching him breaking the promise, feeling betrayed, etc. That's different and a fraught area for couples. The woman usually hates it because it makes her feel like he's saying she's not enough, or being compared to the unrealistic women and scenarios in porn and found wanting. And maybe she's right, maybe he is using the porn to avoid the relationship but doesn't want to tell her that. But sometimes it sounds like what the guy is using porn to avoid is that he doesn't like his life all that much, it's dull or hard or no fun in some other way. If the woman wants the man to stop watching porn, it might be more of a discussion about other things than porn, like what gap in his life he is filling with superficial stimulation, and what he can do to feel more actualized.
Anyway, if a couple is at a rocky spot about porn, and the guy has promised to stop but wants to keep watching, depending on how ingenious he is with excuses, there's also a possibility that he might think it would make his girlfriend feel better about the 'comparison' aspect if he were to cook up a tale that he isn't watching it with the fantasy of having sex with the woman, he is watching it to understand the woman's viewpoint in sex. "I'm not mentally cheating as the man, honey, I'm empathizing with how you feel." I realize that theory is far-fetched, but some guys might give that a try if they want to keep watching porn and their girlfriend isn't buying it.
I think what the above poster said is more likely, either he simply likes to fantasize in all directions or he has some kind of gender dysphoria, but if neither of those is it, you might consider the possibility that he just dreamed up that story thinking it would make you feel better if you thought he wasn't comparing you to the women in porn.
Hi- this is definitely not uncommon. Sexual fantasies can be all over the place, and since your boyfriend is attracted to women, it seems like a reasonable fantasy to get pleasure from imagining how a woman would feel in a sexual situation.
However, there are other reasons why this could be happening. If your boyfriend has gender dysphoria, he might enjoy imagining himself as a woman. Or if he likes imagining himself as a woman being pleasured by a man, he may have some sort of attraction to men as well as women.
It could be a lot of things, but you can't really know without knowing more about it. If he only has the fantasy of being a woman in a sexual way, it is probably nothing more than just that, a fantasy. If it is about more than sexual pleasure, then it could be something bigger.
My recommendation is to ask your boyfriend more about it. What exactly does he like about this? Is it just when watching porn? etc.
It's probably nothing to worry about. So good luck. :)