We were segregating by both race and sex only two generations ago here in the American South, and only a very small minority of people were willing to ask whether the status quo could be justified from first principles.
It seems to me that (as skeptics) we have to take this small minority seriously unless we can come up with an argument in favor of segregation by sex which is grounded in something more than tradition and taboo. What's yours?
Before I answer, I want to discuss something that came up briefly before. I'll explain the connection later. The thing I want to bring up is perverts who want to get into women's locker rooms.
Here's the thing. They aren't perverts. Guys who would like to see inside the girls' locker room are normal. We like seeing women naked.
Now, most of us have empathy, and we know that the women we would be watching don't actually want to be watched, and so out of respect for their feelings, we don't go in there and we don't peep. Sure, we would like to, but it's really a mean thing to do, and we're decent at heart, so we don't do it. For a few of us who lack that empathy, we know there are some combinations of laws and policies that could be used against us if we did go out of our way to sneak a peep. Even if there are no laws in whatever circumstances might exist, we know some of the women carry weapons, and others have boyfriends or husbands, and the consequences are enough to deter the ones where basic empathy is not enough to keep their eyes where they belong. The only ones left, the ones who would actually invade that privacy, either physically, or by peeping, or by hidden cameras, are people with no empathy and no self control, and those people are really creepy.
In other words, it's really creepy to peep, but it isn't perverted. There's a difference.
Now, back to your question. I don't have an answer or even an argument about why places we take our clothes off ought to be segregated. I can't say whether it is societally imposed, or is perfectly natural. I suspect, though, that it is natural, and it is related to an enhanced feeling of vulnerability created by the fact that people really are more vulnerable without clothes. There's also an awkwardness that occurs any time sexuality is introduced into any situation, especially if it is introduced by one party and not by the other, and that would happen more if we spent more time naked around the opposite sex. As noted above, men like to see women naked, and women know that men like to see women naked, and if we spent more time naked around each other, things would be awkward. Would we get used to it, and get over that awkwardness, and all be better off for having shed an irrational inhibition? It's possible. If ever a movement springs up to eliminate sex segregated showers, I won't be one of the protestors carrying signs against it. I think I might like the result. However, the ladies know that I might like the result, and that knowledge does not make them enthusiastic about the possibility.
So, we're kind of stuck with what we have, and it's only peripheral to the thread topic.
ETA: Also, what G. K. Chesterton said, quoted by Ziggurat.