I haven't been there, but I've heard of a place named Stag Run in Mauckport, Indiana that is a private, all male (gay) clothing-optional resort, but their calendar shows several events that look like they're co-ed (female-friendly). We've tried to go there for their HedoFest the past 2 years, but our schedule didn't work with it.
Although Drake's Ridge near Bennington, IN is listed as a resort, it is very rustic and back to nature. The trails are very nice for hiking and the ponds are great for fishing.
I did a whole study of photos on it's natural quality just about 3 weeks ago.