Society has accepted public toilets as places where differences are set aside and equality is the norm. But a major difference still exists: that between men and women.
Public toilets have failed to erase this difference. It is acceptable for a Wall Street broker to take a leak next to a construction worker, or a politician to take a dump next to a cop at an airport restroom; however, it would be unthinkable to take a dump in a restaurant while your date applies make-up -- or worse still, to take a leak while she takes a dump.
Unisex toilets would erase the difference in gender -- and, in consequence, would be people equalizers. They would demystify the Victorian taboo of shame between men and women when going to the bathroom. Urinating and defecating are physiological functions, just like breathing, sleeping, and eating. There's nothing wrong with them. Poop is natural and there's nothing disgusting with it if handled properly. It is society which has attached to it the taboo of uncleanliness, guilt, and shame.
This can be partly understood because, in the 19th century, sewage systems were not always existent and the toilet as we know it today was not yet perfected. There were many outbreaks of cholera and other diseases. So poop was considered extremely dangerous, and it was understandable that people had an aversion to it. On the other hand, Victorian morality made it fashionable to pretend you didn't poop. The higher you were in the social scale, the less "earthly" you were. So for the higher classes, it was essential to pretend they didn't poop and thus hide all the aspects related to pooping. That's why the water closet was invented. And that's how privacy became the norm.
If pooping was considered taboo for society as a whole, for women it was unthinkable. "Girls don't poop" -- and the only thing emanating from a women's body should be the scent of roses. That was the prevailing ideology, and that's the primary reason why men and women are segregated when going to the bathroom. At least in Western society.
Opponents of unisex toilets might argue that they're not soundproof. True, but what's wrong with hearing farts, plops, or crackling sounds? They're not worse than hearing someone blowing their nose. Or sneezing, or coughing.
They may argue that they're not smell-proof. True, you can smell someone's gas, but what else would you expect in a public restroom? And would you really care if the poop smell comes from a man or a woman? Poop always stinks anyway.
They may argue they're not sight-proof. So what's the big deal with seeing someone's shoes -- or high heels -- under a stall? You're not seeing nudity or anything else you wouldn't see outside the restroom.
Finally, the test of fire: they may argue that women might get offended by seeing a man's penis. Regarding this delicate issue: there's no reason why a woman should see a penis inside a unisex toilet any more than a man should see a vagina. However, since men in a hurry sometimes unzip before getting to the urinal accidentally, my solution would be placing the urinals in a separate area where women would be visually protected. That way, this issue would be solved once and for all.
Furthermore, since most people, men and women, enter a public toilet for a #1, pooping issues would be minimized. Most women would use the stalls to pee; and if they pooped, it would not be such an issue for them since they always use stalls anyway. On the other hand, most men would use the urinals; and, given their proposed location, it would not pose any problem.
If a man did need to pass a B.M., he would use a stall, and this would not pose any problems for him. It has always been more acceptable for a man to show his human side than for a woman. And he would be surrounded mostly by women using the stalls who would not care. Women only care about piss on the seat, and this would not be the case.
A unisex toilet does bring some changes. Women can no longer go to the bathroom without the presence of men, and thus cannot discuss women-only issues; but they would be compensated by shorter queues. And if space affords it, a separate women's lounge with a few women-only stalls could be considered (if the restroom is large enough, of course).
Regarding the main stalls, they should all be labeled for men and women; and most, if not all, should have tampon dispensers for women. The few without them should have a men's sign on them, although women could use them as well.
Changes for men would be minimal, since men don't go in groups to the bathroom and don't linger in it like women. Men would tend to wash their hands more, since it would be embarrassing not doing so in front of women. The biggest issue for a guy would be realizing that a girl poops -- and this is something that could be very weird for us. However, a unisex toilet would prepare people to accept the fact that everyone poops, and it's no big deal, just as gender specific bathrooms do between the genders.
Unisex toilets can work better in some places than others. They can be more easily accepted in places where chances of meeting someone you know are very small; places with very open-minded people; places where everyone is just too drunk to care; or simply places where people are desperate to go and no other option is available. When you gotta go, you gotta go.
From the builder's perspective, the main reasons to install a unisex toilet are the premium placed on space, better utilization, and smaller queues, all of which lead to efficiency, better allocation of resources, and economies of scale. Some places may even install unisex toilets deliberately, since it may be fashionable, like trendy bars and restaurants. Others may install them to generate a certain dynamic between people, like some companies (Ally McBeal, for example). Once a coworker has heard you poop, the relationship goes to a deeper level (nothing sexual involved).
In conclusion, unisex toilets can be installed in most places where at least one reason exists to justify them. Men and women can get used to them and accept them without major problems. Pooping is natural and the taboo around it is largely unfounded and obsolete.
In the 21st Century, we should get rid of Victorian ideas that deny bodily functions. Men and women are equally capable, and can be civilized enough to share a common restroom.
This doesn't mean the end of the magic between the genders. On the contrary, it celebrates our differences and highlights the fact that we are all humans -- and humanity should be united.