Regardless of exactly how, it's all because of the fall. We know that much.
I have debated as to whether to share some of my own experience or not. But I think that my testimony can help some who struggle to understand their own issues. And it can help those who don't have these struggles to at least understand some of how this can happen to a child.
As you know, I was born into an extreme form of abuse called SRA (satanic ritual abuse) and also MK Ultra (governmental trauma based mind control). As that was the case, I was traumatized in every way imaginable. And (please bear with me) ...from at least age 2 to 3, I began being trained in how to please men in various sexual ways. This was part of something called "beta/kitten programming."
I had parts of myself that were used in sexual ways by men. But there was also sexual abuse from women (although not predominantly by women). In my regular daily life, you would have seen me as a quiet, even shy person. It was extreme at times to the point that people would ask if I could talk. I was very very cautious of people. Then, at other times, I was normal, talkative, etc.
But if anyone approached me, I backed up a bit to keep my personal space. I was very guarded. On a conscious level, I didn't know why. But if someone approached me and came into my space, my body would feel ...like it was gearing up for violation, or for some sexual encounter. So I kept my space very guarded if at all possible.
Then, when I came to Christ at age 13, some things changed. I was head over heels in love with Jesus. I knew He loved me and I immersed myself in the Word (KJV), prayer, and Christian (southern gospel) music. Hatred was replaced with love. Many of my struggles just faded away or disappeared instantly. And yet, when I was in a church, I experienced very invasive flashbacks of traumas that happened in rituals (inside "christian" churches), including rapes.
There was one woman I came to trust. She prayed with me a lot. And I spent a lot of time in her home. I allowed my walls to come down a good bit. But there were times that (as a 14 yr old) when she would approach me, my body would automatically gear up and respond sexually ...and it horrified me! I knew that wasn't my desire, so why was my body betraying me?! I was grounded enough in the Lord, that I held onto Him and trusted He would take care of it.
Years later, when I began walking through healing of my past trauma, the Lord taught my heart why I struggled in that way. There is something called "body memories." And some have termed it, "the body remembers." Memory is stored in all the body, not just the brain. My mind and body were trained to interpret physical closeness in a sexual way. As a way to cope with extreme physical and sexual trauma, a child will adapt. A mental process happens ... "I can't stop this, so I will have to survive it." Her mind may even create a separate part who goes along with what is happening, or parts who "enjoy" it (pleasure+pain) ...because that is survival.
So I understood that my body responded sexually... not because it was "how I was" ....but because my body was trained to expect sexual abuse/experiences anytime my personal space was invaded. With a revelation of truth and understanding comes opportunity to heal and overcome.
That was long-winded, and idk if it helps or not. But that is my experience and others who have experienced even just "regular" abuse may be able to relate in ways. That is my hopes.