Yeah, I've been there. I thought my parents were arguing about something and I walked in and said 'You shouldn't be arguing, it's late and I have a math test tomorrow'. And saw... yeah. Parents. Having THE SEX.
Yeah.
I can't even give more details because my brain just... shuts down. I suspect it's similar to how people who are on fire deal with the pain, you can only tolerate so much and then... you feel nothing. That's currently where I am.
Never that far, but I was helping my mom empty her dresser to move it, and in the pile of stuff was a "sensual" game, something cheesy like "A special night". She just said something to the extent of "What, you don't think your dad and I have fun still, too?"
I then passed out.