I have never made one of the (many many many) complaining-about-transfic secrets here (honestly, I don't care about all the transfic in my fandom -- it's fine), but since you asked, one fandom I'm in that has a lot of trans fic is Star Trek DS9. Garak/Bashir is the biggest pairing, and FTM Julian is a very common headcanon and shows up in smut where it's mostly used for PIV sex. Relatedly, it is very common for people to use alien headcanons to essentially give male alien characters both a regular-sized dick (if needed) and a self-lubricating hole. I've seen it used commonly for Garak to also facilitate PIV sex with Garak bottoming. Also very common to give a very similar biology headcanon to Ferengi or Vorta characters to have Quark or Weyoun bottoming with a self-lubricating hole. So yeah, there are many boy pussies in my fandom...