We are a couple. We went together on a Sunday, and it was pretty cool because there weren't many people.
We are not naturists. However, naturism doesn't bother us and even less in water or thermal places. We are used to naked Korean baths, although they are normally not mixed, or even German therms, and that's something we're comfortable enough with.
That said, the clients were mostly men that day (we were only 2 women for around 10-15 men), and it makes it a tiny bit less comfortable for the women.
A naturist place is the same as a textile place: there are fine people, and there are creepy people too. That day, two guys were actually quite creepy, one of them touching himself in the hamam looking at the 2 women that were there. At the end of the day, it bothered my boyfriend even more than it bothered me because I tend to not watch what's going on around me. I wouldn't go back alone as a woman though.
The place itself is small, and old. The swimming pool is kind of cold, but it's actually nice because you can get a cold plunge after the hamam or sauna. We didn't use any of the upstairs equipment but we had a check and it looked even older.
I think that place has potential, because there simply aren't many places in Paris where you can go with your boy/girlfriend for 40 euros, stay the entire day if you want, and enjoy a Sauna, a Hamam, a fresh Pool, and showers (+ Massage or Balneo if you pay additional). But with a little bit of investment to make it look nicer, smell nicer and a little bit of privacy added (for example, there are only 4 showers and all really close to each other so you almost touch the other people by your side), that would probably attract more women and it would probably feel more comfortable for everyone.
One thing you may like: The Sauna is separate into 2 areas, a hot one and a very hot one. That is actually quite nice.