The only (nude) mixed bathing places that I’ve ever been aware of in Japan are almost all rustic, rural places. They are not generally well developed and certainly not “deluxe”.
Here’s about as comprehensive a list as you can hope to find for Hokkaido
https://www.food-travel.jp/category/hokkaido
Places where swimsuits are required are another matter.
For instance, there are public baths within JR Gala Yuzawa station (Shinkansen line). As well as the usual segregated baths, there is a bigger area with a variety of small swimming pools, and part of this complex is a heated open air hot tub which over looks the snowy landscapes (in March there are some nice cherry trees nearby). In this pool complex, swimming costumes are mandatory, so it’s a way for couples to have a kind of public rotenburo experience with a mountain view.
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