In all fairness to small businesses, they may not be able to add multiple bathrooms. I wonder if it would be a solution to simply put up a sign saying "ask for key," and leave it at that.
And for those who have been to major events where a lot of beer is being consumed, I can testify that women have no problem going in to the men's bathrooms to use the stalls. When you gotta to, you gotta go, and toilets don't mind which gender sits on them.
You do realize this is also being pushed on showers for pools and gyms/High Schools.
Your post is loaded with good common sense.
The single-use unisex bathroom (with key if need be) is the way to go and covers all bases.
As an aside: I have never heard of a residential (not commercial) bathroom with a urinal in it.