Our obsession with gender and gender roles infects/affects everything, and makes as little sense as obsessing about whether someone is left-handed, right-handed, or ambidextrous. Seriously, why should anyone care about how others dress, how they identify, or who they choose to sleep with?
IME there is substantial correlation between religiosity and an obsession with gender. On my more cynical days I might suggest there is a causal relationship.
I've found a lot of women don't believe men can be "as" bi as women can be, and there's this suspicion that if a guy has ever been (or just wanted to be) with another guy, they must be more into guys than women, and wen don't want to have to wonder/worry about that. I think this is just a reflection of societal implications around male and female gayness being different though, that women are accepted to exist on more of a spectrum (perhaps because attraction to women feels natural to straight men, so it is less "surprising" or foreign to see even other women feel it), while men are considered more binary.
This is purely anecdotal, but I think that's more in line with other unaccepting straight men's reaction than straight women's (especially since a lot of straight women are into/fine with anal too). I think with women it in many ways comes down to the fact that everyone has been conditioned to think that guys are either 100% gay or 100% straight, while it can be sexy or desirable for a women to go both ways.
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