Let me assure you: this won’t be a moral argument.
Ignore all the biases and predispositions about how multi-gender bathrooms are the morally right thing or the morally wrong thing, for I cannot speak for what other people think is right or wrong.
What I will write about is from another viewpoint: economic efficiency, in that generally one larger co-ed bathroom would be more cost effective and economically efficient than having two smaller bathrooms to separate by gender.
First, let’s make some assumptions and hold things constant. Let’s assume that one bathroom combination is one toilet, sink and soap dispenser and costs a total of $140. A door costs $75, and a paper towel dispenser or hand dryer costs an average of $100. Let’s also assume there is one paper towel dispenser or hand dryer for every two bathroom combinations with a minimum of one in every bathroom and every bathroom only has one door.
Another assumption can be that I am talking of larger stores like Target, ones that have male and female bathrooms as well as one family bathroom. The male and female bathrooms have four bathroom combinations each, and the family bathroom has one.
The bathrooms in this store would have three doors, nine toilets, nine sinks, nine soap dispensers and five paper towel dispensers or hand drying machines.
The total cost would equate to $1,985, not including the prices of toilet paper, soap, plumbing installation and so on.
What general stores have their bathrooms over-occupied to the point that someone is waiting? I would say that this is almost never. Therefore, lets assume that at most, only three of the four bathroom combinations in each gendered bathroom is being used at a given time and that the family bathroom is always in use, leaving two open bathroom combinations at all times.
On parts alone, this would create a cost inefficiency of $280.
Instead of this three-bathroom idea, let’s propose we create one larger bathroom. This larger bathroom would feature six regular bathroom combinations, and a larger stall area with one bathroom combination for families to use. It would also contain four paper towel dispensers or hand drying machines. In total, that would be seven bathroom combinations equaling $1,455 and no un-used utilities, causing maximum cost efficiency.
Granted, this post contains a lot of assumptions and does not factor in all the external costs or values, nor does it factor in consumer preferences or externalities such as laws. It also only focuses on one scenario, and different things can happen for different firms.
Still though, having a larger co-ed bathroom would generally be more cost effective than having several smaller bathrooms.