You can get to the onsen in about 4 hours from Fukuoka Hakata Station and there are also direct busses from Yufuin Onsen Town and Kumamoto city that will bring you to the onsen in 2 or 3 hours.
Men have to wear their birthday suit but women are allowed to cover themselves with a towel.
Besides these unisex baths, you will also find a private onsen and 2 large public baths, one with a rose onsen bath.
From Kyoto, it will take you 2.
In 2019 they changed their bathing rules and now everyone has to wear a special bathing suit during their soak in the mixed-gender baths.
The onsen features 3 unisex open-air onsen baths right next to the river.
Something that is usually strictly forbidden in a normal onsen.
Access: It is a 50-minute bus ride from the nearest station JR Aso Station.