It's an issue because there's not really a legal distinction between restrooms, shower rooms, or changing rooms. People who might not care about mixed sexes in wall-divided restrooms might have a totally different take on mixed sexes in a shared high school shower room.Do people shower in high school though? They didn't when I was there, at least not after PE. Teenagers generally don't like changing/showering in front of each other.
In my school we did occasionally (well, at least the guys; I don't know about the girls), but we generally avoided it. Most guys I knew didn't so much care about other guys seeing us nude so much as we didn't want to see other guys nude, whereas women I know have said the female viewpoint is very much the opposite where they don't mind seeing other women but they don't want to be seen themselves. (I recall my mother saying that when she and my father visited the Blue Lagoon in Iceland she was very uncomfortable with the very open locker room environment for that reason even though it was single-sex.)
Regarding unisex restrooms, I thought the one I encountered in Stockholm was a very sensible design with the booth walls and doors going all the way down to the floor and going up quite high. They had red and green lights on the doors to indicate whether they were occupied. Seemed like a sensible enough system to me except that I don't recall there being any urinals. I understand why some women might be uncomfortable with urinals being in there, but really, they are far more efficient for men's needs and they eliminate the very big problem of slobs who whiz on the toilet seat or splatter on the rim (I'm sure most posters here have encountered that at some point, especially in stadium or arena men's rooms). It wouldn't be that hard to have an area off the unisex restroom, perhaps behind a second door, with urinals.
It was a little bit weird to come out of the booth to the sinks and find women there, but that's just because I'm not used to it in this country (and of course I've been to rock concerts when there were more women in the men's room than there were men simply because our lines were shorter). It was no big deal.
I do think there are a fair number of people who would object to some of the noises you hear, even though everybody farts. The men's room at my office is not always a pleasant place to go because some of those guys are astonishingly loud farters. Makes me wonder what the heck they're eating. One of the women who sits near me has grumbled about regularly finding "bloodbaths" on the toilets in the ladies' room. I really wouldn't care to walk in and find that because I wouldn't want to clean that up before I sat down. (But then, earlier this year we had a problem in the men's room where someone took a dump on the floor! That's nasty, period.)
I'm sure I may have mentioned this before: When I was in law school, some of the women complained (justifiably, I think) that in the law library there were two men's rooms (one on the second floor with two shitters and two urinals, one on the fourth floor) but only one ladies' room (on the fourth floor). They had a legitimate beef that they were expected to go upstairs every time. I recall one woman, a very smart lady who later clerked for a Supreme Court justice, routinely used that men's room because it was more convenient. Anyway, the administrators decided the complaint was reasonable and they converted the second-floor men's room into a unisex restroom. But in doing so, they removed the booth walls and doors, one shitter, and one urinal. I asked about that and they said Duke's rules required that if there is a unisex restroom, it must be set up so only one person can use it at a time. They didn't have a response to my point that TWO people could still use it at the same time, but a few weeks later a second sign appeared on the door next to the "Unisex" sign saying "Remember to Lock Door Behind You." I've always wondered whether that was a response to my point or whether someone literally got caught with her pants down.