The first time they are intimate together, Poe discovers that the First Order removed Finn's genitals at some point (whatever age you want, and reason is up to you--as a standard thing they do to everyone, as a special punishment, a way to control him, some fucked up fetish thing, etc.). Finn never warned him beforehand because it was normal to him and it slipped his mind that other people probably don't get their balls and/or cock chopped off.

I am particularly interested in negotiations of the emotional minefield in the moment immediately following this discovery. How do either/both of them feel, how do they think the other person feels, reactions, etc. If you want to write them figuring out how to have sex/do other sexy things around Finn's castration though, I wouldn't be against it at all!