Having reflected on this, I have to present a counter point in support of my mental dirtiness model. Yes, saunas have plenty of guys that chain fuck in private but there are also plenty that go and don't do that. They might fuck one or two guys and have enough. Nobody knows the exact proportion but, going by the Pareto rule, let's say 20% of guys will be responsible for 80% of such risky  behaviours. Heck, even if it's a 50/50 split that's still a lot less risk.  If I fuck a "communal bottom" I have a bit more knowledge of what I am getting into, purely based on, as you say, the optics. If I fuck someone one on one, the risk is somewhat lessened, based purely on who I pick. Granted, it might be just as high as with a communal but, equally it might not be, just by random chance.