I walked in on my parents when I was a kid. I went downstairs for a glass of water, saw my dad's naked ass on top of my mom in the living room in front of the fireplace, and ran back upstairs and cried.
I locked my door and wouldn't let my mom in because I was afraid I was going to be in trouble because I was supposed to be sleeping. I wasn't, and she got me water, and told me that "Daddy was very embarrassed."
As I got older I heard them more than most people hear their parents having sex. It didn't bother me at that point though, because I'd rather know that they still love each other, you know? It's kinda sweet if you can get passed the part that it's your parents and therefore kinda gross.