AO3 statistics for trans men
I looked at three main categories of ao3 stats: number of explicit fics, ratio of M/M to M/F fics, and different types of sex listed under "additional tags" (vaginal, oral, etc). There are definitely more things that could be looked at but these are what I focused on.
At the time I collected these numbers (they've likely already changed, fic authors publish stuff fast) there were 14,221,609 total fics on ao3. Of those, 2,569,913 fics were rated "explicit," or 18.1% of fics. There were 91,487 fics tagged "trans male character," and of those, 43,845 were rated "explicit." That's 47.9% of fics tagged "trans male character."
Nearly half of all fics featuring trans men are explicit, compared to about a fifth of fics overall. This indicates that trans men are heavily sexualized in fandom spaces.
For fics in general, there were about 2.0 times as many M/M fics as M/F fics (6,822,062 and 3,439,600, respectively). For fics tagged "trans male character," there were about 6.0 times as many M/M fics as M/F (68,507 and 11,359, respectively).
When filtering for fics rated "explicit," the difference in ratios is even more significant. Explicit fics in general had about 2.2 times as many M/M fics as M/F (1,616,555 and 743,455), while explicit fics tagged "trans male character" had 8.6 times as many M/M fics as M/F (38,490 and 4,489).
Fandoms in general tend to lean more towards M/M relationships than M/F relationships, but this discrepency is much stronger when it comes to trans men, and I'm not sure what the reason for that is. Trans men are more likely to be non-heterosexual than heterosexual, but not necessarily more likely to be attracted to men than women (a lot of trans men are bisexual, pansexual, queer, etc).
Trans male characters are more commonly shipped with men than women, to a greater extent than the overall fandom preference for M/M ships over M/F, and I'm not really sure why, but it's definitely interesting.
For explicit fics tagged as "trans male character," the most popular additional tag by far is "vaginal sex" at 13,685 fics (31.2% of explicit fics tagged "trans male character"). Since this tag doesn't necessarily refer to the trans male character (it could refer to a cis woman being vaginally penetrated by a trans male partner, for example), I filtered for fics tagged as M/M. That resulted in 12,504 fics tagged "vaginal sex," or 32.5% of explicit M/M fics tagged "trans male character."
Overall, for explicit fics tagged as "trans male character," the most common additional tags referring to different types of sex were:
Vaginal sex, at 13,685 fics
Vaginal fingering, at 9,908 fics
Cunnilingus, at 7,712 fics
There definitely are trans men out there who enjoy vaginal penetration/fingering or receiving cunnilingus, and many of the fics tagged as such might be reflective of that fairly common* experience for trans men. That being said, there is a really heavy emphasis on vaginas and vulvas in explicit fics about trans men, and I find that uncomfortable.**
*I think? Unsure how common it actually is
**BEFORE PEOPLE START MISINTERPRETING ME: I'm not saying I'm uncomfortable with trans men who enjoy using their vaginas/vulvas for sex, or the existence of that kind of fic. I'm saying that fandoms in general tend to focus on trans men having vaginas to an extent that I'm uncomfortable with.