This might sound stupid but some time ago I had this shower thought level theory, that we're getting this influx of transified M/M ships in recent years because the fanbase, who are all teenage girls, like yaoi but they're still at the point where they're too grossed out by the idea of anal sex, so they just make the bottom an FTM and call it a day. And you get bonus points for representation, what a win!
It's not like any of them could write or draw realistic anal sex to begin with, the whole ABO thing for example comes off like an attempt to make male characters work with female bodies (the whole mpreg and having a self lubricating asshole, don't ask) because these people can't be bothered to learn how gay relationships actually work. Kind of like the other end of the spectrum parallel to coombrain men who assume lesbian relationships are exactly like in the pornos that are clearly made for a male audience. [ETA women in general aren't into anal sex at all, but they like seeing two cute boys making out, so trans bottom it is. Again you can compare this to lesbian fetishizers who like watching two women make out but they also want to see some penetration.]