I wasn't asked, I was coerced. I was used.
I was an adult and I just let him do what he wanted without fighting back, just as I had done as a child.
I was struck by how you make the first comment, which (rightly) asserts how abusive the situation was, but then shift to the second comment, which somehow accepts blame for what happened.
There are so many of us who can look back and see, after the abuse ended, times where further sexual incidents occurred. We therefore conclude that while we may have been innocent kids during the abuse, we stand absolutely responsible for whatever happened to us as older teens or adults. Why? Because we were older. We knew we had choices. We could have protected ourselves or said no. We could have stopped it. And so on. We then take the further step of concluding that "responsible" means we are to blame for those incidents".
I think we should reject that argument, and indeed, all that way of thinking. We can all remember how abuse tears up a boy emotionally. That damage continues into adulthood unless he gets the help he needs, and until very recently how would a boy get such help? So it just continues on into adulthood and affects the man that the boy has become.
I remember you talking about that incident when you were 20, and man, I can just imagine how it played out. You were taken by surprise: you were stunned, and suddenly you discovered, "Here I am again". Dale, you didn't "let him do what he wanted without fighting back", you just collapsed, just as so many other young guys in that situation have also done. Little Dale's feelings of hopelessness and powerlessness just flooded over you, and frankly, I'm just not sure what you could have done to protect yourself.
So I hope you won't accept blame for this incident. Accepting responsibility is something else. Yes, we are all adults and must accept responsibility for what we do, but in this case accepting responsibility would mean seeking help and trying to work through your issues, which is exactly what you are doing.
Accepting blame implies that you could have or should have seen this coming, or that you could have or should done something to prevent it or stop it. I really don't think any of that is true.
Looking back at it honestly, do you?