You again pretend I said it would cost nothing more. I didn't say that, I said it would cost more but less so than you are trying to infer.
During construction, adding some pipe to the wall I have already designed for the public restrooms is not difficult, and would not complicate much for the plumbers. This only becomes exceedingly expensive if you were to update an already existing facility. So, plumbing, one of your "objections" just isn't that much more.
Adding entrances, again, not that danged expensive during construction, you design a place to put the door, the door goes in there. It is again only exceedingly expensive if you were to try to update a current structure. Then you assume that one couldn't make these about the size of a stall (most that I have seen easily could fit inside some of the larger stalls in a restroom). All you need is one toilet and one sink and enough room to wipe and wash your hands.
Basically, think of that wall full of stalls in your basic female restroom, think of it expanded a foot or so back having an entrance, a lock, and a small sink inside... While this would be more expensive than building two echo chambers full of strangers, it would not be massively more expensive if you design this space beforehand. The square footage actually becomes smaller in this use as you don't have the huge spaces that folks walk through to get past the sinks and urinals to the stall so they can stink up the room for you....
A well designed space would be minimally more expensive than huge echo filled spaces that are often less than fully utilized and actually gain you more customers because the comfort level increases. You no longer have to share the space with strangers, and you no longer have the issue of small children sharing the space with somebody you consider strange, or the wrong sex, etc.