The fun thing about smutty fanfic is that kinks are weird and nonsensical and often impossible to predict based on someone’s public-facing persona, so most of the time it’s basically impossible to be out of character. Is Batman into petplay? Does Hans from Frozen get off on being beaten with sacks of oranges? Go for it - it’d make as much sense as anything else!
Batman fights crime in a borderline-daddy leather fursuit with only his eyes and mouth visible and you’re wondering if he might be into weird shit.
To be fair, Batman lives in a world where that sort of thing seems to be regarded as normal.
Are you implying that Batman’s outrageous kink would be vanilla sex?
Well, kinks are defined in relation to societal norms.