This was NOT your fault.
Boys do NOT choose to have sex, pose for porn, or get pimped out. You did not have the ability to give consent as an adult would. You were looking for love and attention, and your abusers instead used you selfishly. They did NOT have your best interests in mind.
You were groomed, coerced, and used as a sex toy by adults who should have been taking care of you, instead of satisfying their own sick desires.
Just because you had an erection or ejaculated, it does NOT mean that you enjoyed or wanted it. You body responded to stimulation as it was made to. Thats all.
Just because you didn't say no, or fight against it, DOESN'T mean that you wanted or it or agreed to it. It does NOT mean that you liked it or that it was allright. You were a kid. You trusted adults to do whats right. They betrayed you.
You need to see all this as sexual abuse which you were NOT responsible for, DIDN'T agree to and was very damaging to you. Trust your therapist. He/she is trying to help you confront your past. Not a pleasant task, but one that has to be done for you to have a whole healthy life. Please do it for your own sake.
"Suffering was the only thing that made me feel I was alive,
Thought thats just what it cost to survive in this world,
...now I haven't got time for the pain... " -Carly Simon now 67!