Google Dallas Russian Banya. I went there years ago, and the common areas (shower, sauna, etc) were coed and clothing optional. I was meeting someone for dinner, and ended up staying for the spa side activities, and they gave me a towel and pointed me in the right direction.
What would be the point of going to a co-ed spa? My impression is that the women who go are usually part of an older couple, you'd rarely find singles and the ratios heavily skew towards men