Chastity's Nook

What it says on the tin - any situation where Caleb (and the rest of the Nein?) are put under Geas with the sole instruction be to fuck each other. Maybe the others want to roll the dice (ha) on not obeying, but they're already injured and realize Caleb could literally be one-shotted. Maybe it's a "every day for a month" type situation. Maybe they're trapped in a labyrinth and they all have to find each other and have sex before time runs out! Maybe Caleb's the only one under it and he doesn't tell anyone, and the others don't find out until he drops unconscious. Maybe Caleb has to have sex with someone every day and they all trade off helping out until they can get it removed. Maybe Caleb is determined not to follow the command, almost dies a couple days in a row, and the others decide they have to override his wishes, with lots of self-loathing all around but determined to save his life. Maybe Caleb's trapped with someone else with it, and he has to push past the other person's reservations to save them. Literally any interpretation of it you want.

Any relationships, up to and including polynein.