I frequented the Yunomori near Phrom Phong back way back since 2015.
There used to be some 'going-on's' happening in the toilet back when the cubicles weren't all blocked up. There were a lot of action happening in the urinals as well since obviously, you only have that small face towel to obscure your business. But mostly, the hot and youngish locals would normally go there to show off their goods, cruise, and then 'take-home' their pickings where it is more safe to play.. But now that scene has died down since the staff have become very strict in catching any hanky panky.... I've even witness a local and a tourist getting caught and shamed to death by the staff, telling they are going to be taken to the police.
Alas, that onsen has just become a place to go to and really relax, and just enjoy the eye candies from all over the world visiting Bangkok