So when my younger brother, MJN, visited me, it was during the Beppu-shi Onsen Matsuri (hot springs festival). When we stepped off the train at Beppu Eki that day, we got this photo snapped for free.
We meet up with Jenny and Tomoko (who has a car) and watch the annual burning of the mountain. Afterward we go to an okonomiyaki restaurant. Still in a festive mood, we decide to take a bath(what else does one do during an onsen festival?)
Tomoko drives up into the mountains to an onsen that has a very large rotenburo (outdoor bath). It is segregated by a high fence that cuts the bath in half. MJN goes on the men's side, and we to the women's. We talk over the fence. It's very late and most people are downtown at the festival. Soon there is no one else in the rotenburo. Tomoko suggest that MJN climb over the fence to women's bath. This he does and soon all four of us are laughing and drinking and having a pretty good time. Unfortunately, some other women arrive and freak out. "There's a man in the women's bath. There's a gaijin man in our bath!"
The manager comes running. "Dame! Dame! Tomoko does much explaining and apologizing and we don't get thrown out completely. "No men in women's bath. But if women want to go into the men's bath..." he shrugs his shoulders. So we all end up in the men's bath, which pleases the few men already there.
After this, Tomoko drops the three of us off at my place, and we drink cognac and MJN plays his guitar late into the night.
The onsen might have been Ebisu-ya Onsen or, less likely, Yu no Sato.

Adventures in mixed bathing. Yes, we're really in hot water this time.