Sex is a lot more complicated than male/female. There’s a bunch of different sex characteristics that make up “sex” and people can have all sorts of different combinations of them. If you just use a male/female binary, that doesn’t tell people what sex characteristic you’re referring to. Maybe something affects people based on chromosones, in which case people who are xy but otherwise “female” (like with cais) would go in the “male” category and vice versa. Or maybe something affects people based on hormones, in which case transgender people taking hrt would have to be categorized based on that. If you say “male/female”, no one knows if you’re talking about hormones, or genitals, or chromosones, or gonads, or whatever else, so it’s best to be specific and use language like “people with [body part]”