Hiru wa Soushoku, Yoru wa Kedamono na Kare ni Dekiaisaretemasu.

"Can you have that kind of relationship with a friend?" Yuka Kato has been best friends with Kyo Morisawa since high school, and he's ended up as a novelist with her as his editor. But, when she invites him out to a bar she heard about from a coworker, it turns out to be a swingers club.