Just a few words adding to what JDV has said. The sexual abuse of a child doesn't have to involve any touching at all. If someone tries to get off by luring a boy into a sexual conversation, that's abuse. If someone watches him in the shower or keeps walking into his room when he's likely to be undressed, that's abusive too. Taking pictures of him in sexually suggestive poses is abuse.
I think the key here is your comment that the penetration felt like a violent attack. It was! But so also were the other things he did, even if they felt good at the time. If you reacted in a positive way physically, all that means is that your sexual plumbing was working as it was meant to work. Everything he was doing was harming you in one way or another. The penetration was different because maybe at that point you were beginning to see that something wrong was happening.
But none of this was your fault and it was never your task to understand what was going on. I mean, I had been abused for two years before, at the age of 12, I figured out that what the abuser was doing was sexual. You were a kid, after all. You had every right to be innocent and unaware, and the job of adults in your life was to protect that right, not exploit it.
Nobody living can ever stop me
As I go walking my freedom highway.
Nobody living can make me turn back:
This land was made for you and me. (Woody Guthrie)