Poe is captured by the first order, has just been interrogated by Kylo Ren and has no idea what's going to happen to him now. It's the absolute worst time for him to go into heat.
Finn really wants to rescue this man, but he really can't leave with Poe like this. He would never be able to concentrate on flying while he is in heat. But he also can't just leave him here. With Poe's heat getting stronger each minute one of the troopers (or Kylo Ren for that matter) is bound to force themselves on him.
He has to make a decision. He will mate with Poe, end his heat and get the hell off this ship. He is doing that because it is the right thing to do (also Poe is smelling reeeaally good and literally begging Finn to mate with him, but he is trying to not think about that)
The sex itself is very nice and Poe very responsive, but Finn is aware that that's probably only the heat speaking. Once they're finished they hijack the next tie-fighter and flee only to crash on Jakku together. What ensues is pure awkwardness. Sure that stormtrooper just saved his life, but they also bonded for life while Poe was completely out of it. They'll have to figure out how to go on from here. Being bonded for life and all.