Poe has feelings for Finn, but he knows how much Finn and Rey adore each other, assumes they are together, and tries to be happy for them. Then Rey has to leave to find Luke Skywalker and he's spending lots of time with Finn, he blames himself for wanting more and tries to bury the feelings as deep as he can, but then Finn kisses him and he can't help but kiss Finn back. It happens again and again and often enough to resemble a relationship, but Poe believes Finn is only using him as a substitute for Rey and it's okay. Really. He's okay.

Then Rey comes back, Finn is delighted, and Poe starts avoiding him, assuming Finn is going to leave him for Rey. he's been mentally preparing for it since they first kissed, but it still hurts.

Cue Finn being very confused/hurt that his boyfriend is avoiding him (bc ofc Finn adores Poe and is totally in love with him)
Rey is very confused at first, too, but figures it out before Finn does (Finn just assumes it was his fault, maybe he did something wrong and poe's angry idk) and tries to talk to Poe.

Happy ending please (maybe it could turn jedistormpilot in the end if that's your thing)