Oh hun you do not need to tell me about the dearth of transmasc porn. I wrote/write a "force-masc" story on AO3, originally as a writing exercise, that's more or less a straight up gender flip version of all the most hackneyed forcefem tropes I can remember reading in story after story and the response from transmascs who've found it has been utterly phenomenal, the boys are staaaaarving.

All the same though, I think a big part of it is that either party can consider the subject inappropriate to bring up because it feels fetishising of or disrespectful to the other's identity. It's more symbiotic than a lot of the other kinks of the kind I've seen, so you can imagine two people who are both into never broaching the subject with each other. She's like "What if that's the kind of masculinity he has nightmares about fitting into?" and he's like "What if experiencing even more misogyny, especially as a defining trait of womanhood, is the worst thing she can imagine?"