It isn't. It's a weird sex thing. Weird sex things are always opt in. BDSM AUs are common as fuck and they are also not fine to give to a blank sign-up. Coffee Shop AUs are common and many people hate them and they are perfectly fine to give to a blank sign-up anyway because they are not weird sex things. (Does BDSM Omegaverse Coffee Shop fic exist? I would read that. But also I would ask for it.)