You are taking it too literally.
Some genderbent characters are meant to be the same character who was like, hit with a genderbender ray, or something like that. Alternatively, they can be a part of an AU where they were born as the opposite sex, maybe even an AU where everyone is genderswapped. Either way, their personality can differ from canon, since they were either a different character all along, with different name, life experiences and such, or their personality was influenced by the in-universe sex change. Men and women are different after all.
R36 is often a fetish thing, but it doesn't have to be, some people just like to do redesigns and think up "what if" scenarios. Not all genderbent characters are male either, as evidenced by this thread. Are you as triggered by masculinization of female characters? What about canon genderbent versions, like boy!Star or opposite-sex clones like Dani Phantom?