Is Jeju co-ed? Do they have a coed sauna?

There are some saunas that are, like the charcoal sauna

Not really. The pool and dining area is, but that's it.

No, the sauna or wet area, where the steamed or hot pools are located, are not co-ed. There is one lap pool that is coed. There are a set of treatment rooms that are coed. These huts are inside the building and are very nice. They are covered with tatami mats and the rooms are lined with coal, or salt, or semi-preciuos stones (each are independent of another). Ech room is appropriately marked for what each can be helpful for.

The sauna areas are co-ed. The locker rooms and whirlpool areas are single sex.