I did not abuse kids but initiated a few affairs/relationships in my adulthood (>20) with other willing adults that shook the life and even identity of others, quite a few long-term. Now, retrospect, I should have controlled myself better. But I did not and they did not, either.

I've just read through some of your threads on other subforums. What stands out to me is not really your "bad fantasies", behaviors, or whatever you witnessed, but your guilt and shame about it all. Maybe the latter (guilt/shame) might be a better focus for your self-work than trying to figure out where the thoughts come from?