So Poe has both sets of parts down there and it's like an incredibly rare occurrence in his mostly human bloodline? He's been hiding it even as he and Finn get closer and closer, except that when Finn puts the moves on him he has to come clean. He's totally weirded out that Finn will be grossed out, like some people have been in the past, but Finn being Finn is just incredibly fascinated and wants to see how Poe's boy cunt works and dives right in with incredible enthusiasm.