First Order Stormtroopers are given drugs to suppress libido, and they take awhile to wear off. For Finn, this happens after he recovers, and he's not prepared for getting aroused at all sorts of inconvenient times, like when he sees Poe working on his X-wing, or when Poe greets him with a hug, or when Poe snuffles in his sleep (they share a room now), or when he dreams about Poe just kissing him already.

Finn tries to hide it - he's not sure if Poe's interested, after all - but it's hard to hide something like that from someone you spend most of your time with, especially if you wake yourself up one night by coming during a sex dream while moaning said person's name and realize that he's awake and heard everything.

As it turns out, Poe's interested - very interested.