In the eastern surface wilds (I think it's east, opposite Neko's home), there's a little makeshift shelter that eventually has a big gorilla in it. Advance quests until he's there, then talk to the nearby soldiers for a quest involving him. Use the rewarded relic to enter the rat's nest next to him, and you'll find the key to Futa content in there.
Other than first getting it, the only scene I've seen with it is in First Stratum, where a dwarven housewife in the Inn wants some help.