My partner and I are Naturists and as so have been to many saunas both UK & Europe, always mixed and have never seen any overt erotic behaviour in over two decades. In NL there’s occasionally older children in them too at campsites & clubs.
Of course (rarely) a person may show slight signs of arousal, we are humans of course, but you shouldn’t be looking too much anyway. If that does happen enough for people to notice, then just adjust the towel for a moment to hide it until it passes.
Have only been in a ‘clothed’ sauna once, at a hotel in Newcastle-Upon-Tyne, the spa was FOC but as neither of us owns a costume, we just used a towel each wrapped round when walking around and took a chance. The sauna was so dark we got away with undoing the towels when seated anyway.
The jacuzzi proved more challenging, especially for my female o/h, but also managed that by doing the towel dance when getting in/out and keeping well under the water when anyone went past. In comparison to Naturist facilities, no-one joins you when you’re in, they wait until it is clear.
*I will respond on the other thread about when we’ve had an our own ‘playtime’ in exclusive situations. We’d never want an encounter with strangers or to be watched.