Why do some people think Kinosaki might secretly be a woman?

Keep in mind I don't mean this to shit on anyone's theories, this is a genuine question because I'm confused and want to know what people think. I'll be mentioning manga spoilers so preferably I'd rather this question be answered by people who are caught up, but any perspectives are welcome of course

So why do some people theorize that Kinosaki might secretly be a cis woman, or are worried that he'll be revealed as such? Not only is he fully confident in showing his naked body as 'proof' right at the beginning of the story, but also >!in the hot springs chapter, although his towel is held up to his chest, it's clear he doesn't have breasts when we see the scar. !<Putting aside any discussion of Kinosaki secretly being a cis woman being unecessary to lie about>!or not fitting the story's queer themes as strongly,!<I just don't see any in-universe indication that he's secretly a cis woman. Is there something I'm missing, or do people just feel that Gero would only end up with Kinosaki in a heterosexual way, à la Mulan or something? What do y'all think?