When i was about 12 or so, i woke up early one morning to the sounds of my parents having sex.
I tried everything i could as i sat there to just go back to sleep, or drown out the noise, but i stayed up for the whole thing. (I was petrified of getting up and making a noise that would tip them off to the fact that i was awake. Didn't want any awkwardness i supppose.)
Afterwards i noticed that my dad went to the bathroom and showered, and then would get fully dressed for work that day.
A light went on in my head because the majority of the time when i woke up in the morning my dad would still be in his pj's, but i had noted that there was the odd occassion when he would already be dressed and ready to go.
After that day, i knew that on days where he was dressed already, that meant that they had had sex.
It definitely had an affect on how i developed. To this day it still gives me the heebieejeebies.. The sounds that my mom was making.....:\