It’s like the discussion about if you can be racist toward white people.

Depends on whether you’re using the common definition, or intersectional feminist jargon. That is literally the distinction.

Common definitions:

prejudice, discrimination, or hostility directed at people because of their race or ethnicity.

Intersectional feminist jargon:

Racism is racial oppression embedded in social structures and power relations, often operating together with patriarchy and other systems of domination.

PS: Race isn’t real. Not even a social construct: it’s straight up an arbitrary line drawn by a bunch of ignorant assholes, for the sake of being assholes. Ethnicities are real though.

It’s not. You’re boring and stupid. Go read a book.

It isn’t. My sex is XY, but I would be allowed entry because I’m a (trans) woman. More importantly, it’s about the shared experience of being treated as a woman or being at risk of being treated as a woman in patriarchal society.

That said, the social status of someone who is known to have XX chromosomes or who is at risk of people learning they have XX chromosomes would also have a lot of overlap with women, so it would make sense in many cases to categorically allow everyone with XX chromosomes even if they are (cis) men.

(I don’t think cis men with XX chromosomes are medically possible, but you can have cis women with XY chromosomes because testosterone insensitivity isn’t lethal).

So these days, most of the time you would have more queer-inclusive categories like FLINTA, explicitly including everyone who has experience with patriarchy as “the woman”. Different exclusions make sense in different situations; sometimes it makes sense to exclude trans men, sometimes it makes sense to exclude people who don’t menstruate. Sometimes organisations are wrong/immoral about who they exclude, like TERFs. But a feminist meeting without men is going to be able to touch on a lot of topics they otherwise couldn’t safely and go a lot deeper than when having to explain things to men.

(There’s also the “sexism = prejudice + power” thing, which I don’t really vibe with as a rebuttal because it neglects the power of local institutions that may run askew from larger society; if you can host a conference, you have enough power for your prejudice to be sexist).

Yes. Because sex discrimination and sexism are not the same thing. And not all forms of discrimination are wrong. We just use “discrimination is wrong” as a useful first-pass mental hack. But every non-discrimination law is written with certain reasonable exceptions built into it. It is sex discrimination, but not sexism, to refuse to hire cis men as wet nurses. It is disability discrimination, but not illegal discrimination, to refuse to hire someone in a wheelchair to be a circus acrobat.

This is sex discrimination, but it is neither illegal or sexism. It’s not saying that men are inherently inferior to women. It’s saying that there is a bona fide reason to make this space woman-only, based on the lived experience of men vs women.