I don’t think people comparing “femboy” to slurs like the n-word are wrong per se, but if we want to analogize between the dehumanizing influence of fetish terms used towards transfems and black people respectively, words like “stud” or “bull” seem like much more apt points of comparison.
They’re all identity terms arising out of deeply dehumanizing sexual practices that are still fairly ingrained in the superstructure of prolific fetishes. For femboy, we’re talking about a porn genre centered on the eroticization of third-gendered subjects, which has been used at times to degender and closet trans women; for bull/stud, we’re talking about “blacked”-style raceplay and the fetishized “virility” of black men, which has a long and sordid history dating all the way back to American chattel slavery (both stud and bull reference the terms for male livestock breeders among horses and cattle; slaveowners referred to their slaves like literal farm animals).
They’re all terms which are still held up as aspiration to some degree due to their embeddedness in the superstructure of prolific fetishes. Femboys/studs/bulls are all commonly objectified, dehumanized, commodified. But leaning into these identities can sometimes mean being a hot commodity, and that can be an attractive prospect to some people.
The aspirational nature of these identities extends even outside of the marginalized groups that they harm. If being a stud/bull means being exceptionally dominant and virile, then non-black people who want to want to be those things will adopt those terms for themselves, decontextualized from the wider history. TME people who are attracted to the idea of looking like an estrogenized twink will do the same with femboy, equally ignorant to the wider context.