Basically once Finn comes out of medbay and no longer has whatever chemicals running through his body that the first order stuffed him full of, they learn he's a humanoid species that use their really aggressive pheromones to attract compatible mates. Finn is bad at controlling it, especially when Poe is around, and it spreads to pretty much everyone in the vicinity, making the base hot for Finn. Meanwhile Finn is doing his best to hide from Poe because that is embarrassing and he doesn't want to pressure his first friend in the world into doing anything.

Except Poe doesn't feel any differently for Finn than he did the first time they met. (aka he's already completely in love and the pheromones aren't changing a thing.)