I can actually give one perspective on this as I can relate.
I'm presuming that for him, it's likely a fetish (I could be wrong, only he could say for certain, but I can at least give a perspective if it's a fetish). It's a discussion you'll have to have with him at some point.
Fetishes are something I struggle quite a bit and actually find a bit distressing as the fetishes were imposed on me and not by choice. There's a recent youtube video that does a good explanation of the science behind sexual arousal through fetish:https://www.youtube.com/watch?v=-h2xWLbXkT8
(Fairly SFW video, surprising given the topic).
I'll give you my story -- (TW: CSA story).
My grandfather was an aggressive child molester with a strong taste for young girls. Unfortunately for me, I'm a boy and he was always very angry about that. Basically told me it was my fault for what he was doing and I was a secretly a girl. On a few occasions he forced me to crossdress before raping me and saying "You're a girl. Girls are for penetrating."
As an adult, I am straight. I don't find men attractive nor arousing. But... (And I'm terribly ashamed to even type this out...) because of what happened to me, I do find strap-on play (Woman wearing a dildo) terribly arousing. I find gender role reversal items arousing. Likewise, I don't have any gender dysphoria it's just... what I was taught. It's stamped into my brain. I find women and the female form very sexy (My wife in particular
), but there's also this part of my brain that says "You need to pleasure a phallus too." It doesn't have to be a part of the sexual encounter (Wife actually has zero interest in the fetish, but that's another story.) but it is something that I respond to. That's the troubling bit with porn is it is remarkably powerful to our monkey brains.
I hope that helps a little bit. This might be something your boyfriend doesn't fully understand himself. It might be just a thing for him to indulge in privately, or it might be something you can support and join in in a limited fashion. Just please don't judge and condemn him for the kink; we (survivors) have a lot of scars that are very strange and run very deep.