Maybe it's because his mother died so soon or because his father was working a lot so they didn't have the time to explain it to him. maybe he overheard it in a bar the younger recruits frequented.
Poe is convinced that what happened to him was his own fault. He was being too flirty, smiling too much, hell, he was half-naked doing repairs on his x-wing, anyone would do what the guy did.
But it happens again. Sometimes it's the same guy, sometimes his friends. Poe's squad is joking about his apparently very active sex life. They probably think he's a slut. He kinda is, isn't he?
In the aftermath of the destruction of the Starkiller there are a lot of things to be done. None of the guys come for him. Maybe it's finally over. Maybe he is not giving the wrong signals or is not behaving like a whore anymore.
He starts feeling safer, grows closer to Finn. And then they come back and he doesn't understand why. What has he done wrong this time?
Finn is the one who finds him after the guys leave. He listens to Poe when he breaks and tells him everything. He looks shocked. But for different reasons than Poe assumed. Because you can say what you want about the first order, but they took rape seriously and they accepted no victim-blaming, even from the victims themselves. So Finn will do everything to help Poe to stop blaming himself.
(bonus points if finn and rey track down the abusers and bring them to justice)