Honestly? A lot of genderswap reads like the author wanted to write a fanfic with the premise of "what if I were [swapped character]?" The most extreme example I've ever read was an Iron Man fic where Tony was secretly Toni, and secretly 19, and secretly all that booze was really just apple juice, but there are plenty of fics in plenty of fandoms that are just as self-indulgent (if less obviously written by a younger teenager). Personally, I'm fine with it a lot of the time because if the story is good enough I don't care if John Watson is suddenly Joan Watson with a bunch of oddly specific hang-ups and fetishes that probably line up pretty well with the author's. But, I absolutely get why someone else would read that fic and be annoyed.