TLDR: From a very early age I had an understanding of what explicit sex was, even if not a full understanding of the contexts in which it might occur, or the full reach of what it can mean.

When I was three years old, I stumbled upon my parents snuggling under the blankets in post coital bliss. Nothing was exposed, and they were just laying there talking quietly to one another. My dad noticed me right away, and I'm sure more surprised than anything else, cried out, and while my mom was laughing shouted for me to get out - "It's too cold in here for you! Get out!"

Giggling madly, I took off quickly. I had no concept of sex at that time - but somehow I knew what they had been doing. I knew that they'd been doing something special that they liked doing, something that they did as parents and as a happily married young couple, something that did not involve me.

Then, at the age of five, I once had a particularly poor dream. I don't remember what it was, but while I was a fairly independent child and required some combination of thunderstorm and boogeymen to even begin considering the concept of going to my parents, I had risen from my bed seeking them out.

The hall light was off, and I slowly opened the door to their room afraid of waking them up.

There was light coming in through the window, and I saw them having sex: my father was on top, and they were just going at it. I stood for a moment outside of their room, my hand on the doorknob, and in the quick instance between seeing it and stealthily closing the door again, I understood completely. This was how children were made. This was something they did recreationally - that is, not every time they did it made children. It was natural and part of their being adults.

Completely shaken out of the fear I'd felt from my dream by the sight and sudden total grokking of what I'd encountered, I simply went back to bed.

I do not know why I knew, or why I was just okay with it, and sometimes wonder if it is a common experience to have dealt with the sight of my parents getting it on by accepting it and completely not caring about it. More often, however, I wonder why the idea of 'sex' was so easy for me to understand - but the concept of 'abuse' took so long to sink home.

Denial can be a powerful thing, I suppose.