Sanitorios at Mercate This is probably only one of many public bathrooms that are attached to a mercado where there is a ton of action. Completely hidden, you'll have to search around to find it
It is in the food stall mercado across the street from one of the big mercados on the corner of Calle de Ignacio Aldama and Calle de Miguel Cabrera. I am sure that no one outside the locals know about it, so it is going to be full of local dark skinned horny uncut Mexican guys. Numerous stalls with heavy metal doors, and barely any room inside the toilet stall to move in. Stand up and see who else is looking over at you, and you can take it from there. Very covert action, so it got a little frustrating. It is a public bathroom after all, but the hunt was pretty fun when I was there. Stalls are facing one another, so you could try to just leave your door open and see who looks in, but be discreet. Again, this is Mexico, and a very conservative place.