I think each of us has a different story and explanation, but my own understanding from my own experience, is that as an adult, one can look back and fully grasp what was going on. Many of us only feel the affects (or acknowledge them) years after the abuse when we are older, but at the time of abuse we were sexually charged, beginning puberty and expressing our sexuality-- however it was taken advantage of by someone older and often with some form of percieved authority, wisdom, trust etc.
The idea of youth and their sexuality makes me very uncomfortable too and indeed it is a trigger to us for obvious reasons. You are reminded of that innocent, curious sexuality you experienced and now you understand either the perversity or harmfulness of how it was manipulated by someone else for their own personal benefit, leaving you with emotional and psychological scars.
Hang in there my friend!! Have you begun to look for ways to reduce the impact on triggers or how to calm your mind down upon being triggered?