Finnish sauna enthusiast here: going to same sex sauna naked is absolutely common (although you'll also find people who avoid it), but mixed sauna is less commo

Finnish sauna enthusiast here: going to same sex sauna naked is absolutely common (although you'll also find people who avoid it), but mixed sauna is less commonly done. Amongst technical university students it used to be effectively universal that sauna was all mixed and naked. At some point it became common that some people, females especially, would wear swimsuits into sauna. It has also become common that before mixed sauna there is scheduled separated sauna turns for those who want to go but don't want to enter mixed. (Scheduled so no one has to raise their hand up and publicly say "I'd prefer to go without mixed bathing.") In private life, a mixed naked sauna between eg. close friend families is something I consider pretty normal and do frequently enough without really blinking an eye. But that's because it's between two sauna-crazy families. It's all pretty dependent on the person or people.