Your parents have sex. Even if they’re old, or not together -- and it's not with each other -- they're still having it... maybe even right now (sorry). If you've been as lucky as they have, you haven't caught them in the act, but that doesn’t mean you’re in the clear.
You may have accidentally walked in on them when you were growing up, but catching your parents as an adult? Well, that will make you wish you were too young to understand. Here's how to cope...
Exit the situation
This may seem obvious, but it's very possible you'll just stand there, unable to think or move, brain not comprehending what your eyes are seeing. Pull yourself together and exit the room like it's on fire (it kind of is). This is not the time or the place to freeze up, so GTFO ASAP.
Don't take it personally
All parents are different, but yours are probably not some exhibitionist creeps who wanted to get caught. You're not a victim for catching them, even if you might be traumatized momentarily. What happened has nothing to do with you; everyone is embarrassed here.
Refuse to address it
Your parents don't know how to handle this either, so they might make the mistake of trying to talk about it. They'll attempt to make themselves feel better about the situation by joking about it, or worse, asking if you have any questions. You'll only ask one: "Can you not?"
The same type of absurd explanations parents offer their kids when caught -- they were just playing their special game, or doing grownup CPR -- are going to come in handy now. Instead of watching them struggle through an explanation, just rattle off some b.s. about how you were sleepwalking, in the middle of the day, when you came by to do your laundry.
You all have reasons for why it didn't happen, even though it totally happened. Your parents don't have to believe you, and you don't even need to believe yourself, but everyone welcomes a little context.
Talk about it with your friends
You can't talk about this with your parents, because it's too gross, so you have to open up with other people. Your friends will all lend sympathetic and disgusted ears. Maybe they've gone through something similar, and you won't feel so alone after hearing about it. Or they'll just make fun of you (and your horny parents), and laughing along with them will help.
All right, no more denying it. Your parents still make love. And that's a good thing! No one wants to actively think about it, but in a big picture sense, sex is a healthy part of adult relationships. That doesn't make it any less shocking to catch a glimpse of, but you can appreciate the greater good behind it. You want your parents to be happy, right?
Applying what you've learned
Now you know you can get through any mortifying situation, but more importantly, you've learned that parents are people too. Many adult children (myself included) walk around their parents' homes like they own the place, even when they don't live there anymore. Your parents can do whatever they want now that you're grown up, so when you really think about it, why wouldn't they?
Respect your elders enough to assume they're still enjoying themselves -- and then always knock first.