"You look/act gay"

I never understood the whole "you look gay", "you can't be more gay", "the gayest straight couple". What the hell is it that makes someone look or act gay then?

I've seen that both in cases of real people and fictional characters where people just say that even if they are straight (then they just say something like "the gayest straight couple" or "straight with extra steps" or something like that)

I can somewhat see if it's someone who looks extremely unconventional and stereotypical (kinda), but sometimes I just see a woman with an undercut and people just assume "oh yeah definitely a lesbian" like wth? So a straight woman can't dress a certain way or else she's a lesbian?

Same with femboys, everyone I guess just assumes they are all gay for some reason, even though I don't see tomboys being all assumed to be lesbians (or at least not to the same extent)

I have personally been called gay (never to my face but still) despite being straight because I don't want to dress like everyone else (and I don't even consider myself too flamboyant or feminine). I want to find my style, but apparently that makes me a homosexual all of a sudden. Doesn't that just reinforce stereotypes that were used o stigmatise theese groups in the past?

I literally have been called out by my angry and dissapointed sister that I can't bring a paper fan in a heatwave because "that makes you look gay", even though she had no problem with my dad doing the same because "he's married so it's different" like wtf?

This might be because of me growing up being told to never judge people based on appearances but it still annoys me. Yes I'm not dumb, I know that first impressions and looks can innfluence a lot on how people percieve you, but at that point what isn't gay?

And for reference I've seen both people who do it in a hetefull "you're too gay and therefore not a real man" and the "yas queen" type of assumptions, where people either assume people to be gay and judge them for that or assume them to be gay and applaud them for that despite that not being the case. Both of them annoy the hell out of me just to be clear.