What To Do If You Catch Your Parents Having Sex

Step 1: Hearing it

Hearing your mother in the throes of ecstasy can break the soul, so drown it out at all costs. Use ear plugs, strap pillows to your head, or put on some headphones, a blast of The Village People works a treat for me. Or just slice off your ears.
Find a way to let them know you are aware of their love play. Try playing the same song every time. May I suggest Salt 'n' Peppa's seminal classic "Lets Talk About Sex". Failing that, march up and down yelling Who's the Daddy? and see how they like it.

Step 2: If you walk in on them

The best idea is just never barge into their room If you fail in this simple task, just act innocent. Strike up a conversation. "Hello fine fellows, what are you up to?" They will probably make an excuse, such as scaring bed mites. Be prepared for a straight answer though, in which case the only choice you have is to accept it,
You could just stand and scream. Scream and scream and scream. Until they promise they will never dare look at each others sinful skin again.

Step 3: If it's kinky…

Grab a camera and take a photo, providing you bribes a go-go. Suggest if they ever behave in such a way again this photo will be going up at the work place, in their friends inboxes, and be part of an extensive leafleting campaign.
Sabotage any potential, er, "equipment". Hide the keys to the handcuffs, and smear deep heat liberally over everything
Failing the above, just try and out weird them. Pull up a pew and provide a running sports commentary. They won't be in a hurry to do it again.

Step 4: Not sure if they saw you?

The tension will be unbearable if you don't know if they saw your prying eyes or not, but keep your trap shut and hope the awkwardness dissipates. You should never ever utter the sentence "Father I saw you penetrating ma ma"...Oh god I think I'm going to throw up!
If you can't manage this, just move the hell out and sever all contact. Definitely don't try to eat them.