Bathroom laws
First, let's make clear that the commonly stated reasons for anti-trans bathroom laws are nonsense. "Lawmakers [in North Carolina] had said that they were trying to prevent men from dressing as women to enter bathrooms and commit assaults." (NY Times) Never mind the lack of evidence that this has ever been a significant problem; does anyone seriously expect that somebody who's willing to cross-dress in order to sneak into an opposite-sex bathroom in order to commit a serious felony will be stopped by a local ordinance or a misdemeanor law against entering that bathroom?
That said, there is a taboo in our society against men (by whatever criterion) in women's bathrooms, and women (ditto) in men's bathrooms; why? It's not an Ancient And Honorable Tradition: I haven't done a lot of research on it, but I suspect there was no such thing as a multi-user, single-sex bathroom until a few hundred years ago, and most of those were single-sex by virtue of being in single-sex religious or educational institutions; the opposite sex weren't supposed to be in the building, so there was no need to exclude them from the bathroom. The notion of multi-user, single-sex bathrooms in public places, I'm guessing, dates back maybe 150 years. The taboo doesn't apply at home, where most people have single-seaters that serve whoever gets there first. Some people are bothered by excreting in the same bathroom where an opposite-sex stranger has been recently or might be soon; others aren't. Some people are bothered by excreting in the presence of someone they know, even a sexual partner; others aren't. What really bothers people is being in the same room with a stranger of the opposite sex at the same time while one or both of you is excreting.
So why does "of the opposite sex" matter? One answer is "I don't want someone who might want to have sex with me to see/hear me more intimately than I have offered." Related but distinct: "I don't want someone I might want to have sex with to see/hear me more intimately than I have (yet) offered." In both cases, the "opposite sex" part follows only under the assumption that everybody is heterosexual. If you accept the fact that a substantial percentage of the population is sexually attracted to people with the same-shaped sex organs as themselves, no sex-organ-based or chromosome-based bathroom law will ever protect you from these concerns; the only safe solution is single-seaters. (Multiple well-insulated stalls sharing a sink is a possibility, and my employer has some of those, but many people seem to object to it on the slippery-slope theory.) So in a way, bathroom laws are just another way of desperately clinging to the fantasy that homosexuality doesn't exist and that "gender" is a nice clean concept in which chromosomes, physical sex organs, other physical and emotional attributes, clothing, social and economic roles, and interest in other people's sex organs all correlate perfectly.
I guess another answer would be "to prevent children seeing a kind of genitalia they haven't already seen on themselves." I'm not sure what end that is supposed to serve, unless you subscribe to the theory that the sight of a female body turns adolescent boys into volitionless rape machines, but realistically, anybody over the age of eight today who hasn't seen at least pictures of opposite-sex genitalia isn't trying.
An argument could be made specifically against men (physically larger and stronger, on average) in women's bathrooms, on grounds that they're low-visibility places where one might commit rape without witnesses. Of course, such a law won't prevent male-on-male rape (less common) or female-on-female rape (even less common), so at best this is only a crime reduction argument; to really prevent public bathrooms being used as safe spaces for rape, you need either single-seaters or high traffic. And, as mentioned before, the laws try to prevent a felony by wrapping it in a misdemeanor, which seems unlikely to have much effect.
That said, there is a taboo in our society against men (by whatever criterion) in women's bathrooms, and women (ditto) in men's bathrooms; why? It's not an Ancient And Honorable Tradition: I haven't done a lot of research on it, but I suspect there was no such thing as a multi-user, single-sex bathroom until a few hundred years ago, and most of those were single-sex by virtue of being in single-sex religious or educational institutions; the opposite sex weren't supposed to be in the building, so there was no need to exclude them from the bathroom. The notion of multi-user, single-sex bathrooms in public places, I'm guessing, dates back maybe 150 years. The taboo doesn't apply at home, where most people have single-seaters that serve whoever gets there first. Some people are bothered by excreting in the same bathroom where an opposite-sex stranger has been recently or might be soon; others aren't. Some people are bothered by excreting in the presence of someone they know, even a sexual partner; others aren't. What really bothers people is being in the same room with a stranger of the opposite sex at the same time while one or both of you is excreting.
So why does "of the opposite sex" matter? One answer is "I don't want someone who might want to have sex with me to see/hear me more intimately than I have offered." Related but distinct: "I don't want someone I might want to have sex with to see/hear me more intimately than I have (yet) offered." In both cases, the "opposite sex" part follows only under the assumption that everybody is heterosexual. If you accept the fact that a substantial percentage of the population is sexually attracted to people with the same-shaped sex organs as themselves, no sex-organ-based or chromosome-based bathroom law will ever protect you from these concerns; the only safe solution is single-seaters. (Multiple well-insulated stalls sharing a sink is a possibility, and my employer has some of those, but many people seem to object to it on the slippery-slope theory.) So in a way, bathroom laws are just another way of desperately clinging to the fantasy that homosexuality doesn't exist and that "gender" is a nice clean concept in which chromosomes, physical sex organs, other physical and emotional attributes, clothing, social and economic roles, and interest in other people's sex organs all correlate perfectly.
I guess another answer would be "to prevent children seeing a kind of genitalia they haven't already seen on themselves." I'm not sure what end that is supposed to serve, unless you subscribe to the theory that the sight of a female body turns adolescent boys into volitionless rape machines, but realistically, anybody over the age of eight today who hasn't seen at least pictures of opposite-sex genitalia isn't trying.
An argument could be made specifically against men (physically larger and stronger, on average) in women's bathrooms, on grounds that they're low-visibility places where one might commit rape without witnesses. Of course, such a law won't prevent male-on-male rape (less common) or female-on-female rape (even less common), so at best this is only a crime reduction argument; to really prevent public bathrooms being used as safe spaces for rape, you need either single-seaters or high traffic. And, as mentioned before, the laws try to prevent a felony by wrapping it in a misdemeanor, which seems unlikely to have much effect.