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).