Since OP said their stairs creak, probably the parents already knew it. But they had to make choice as well- if they pull it out immediately then OP would know they had known it- that would be even more awkward afterwards. So they just pretended not knowing OP see them and continued. If I were OP, I would just back up.

It may have been weird and awkward but you live under their roof where they pay the bills, I'll be damned if anyone told me what I can and can't do in my own house.

so now u know how you invented

You should have strutted right through and said, "Good morning!"

Hey OP, maybe when it's way less awkward you could talk with them about it? Tell them it was very inconsiderate because their child was home and they got freaky in a public area in the house.

i woulda yelled gotcha! and ruin the mood

The appropriate response is to run--either back upstairs, or out of the house and far away. By the time they realize what just happened, you'll be gone.

Hey everyone! I loved reading your comments. The good, the bad, and even the ugly. Let me just clear up that it was my moms birthday, so I came down early to wake her up with breakfast. I wasn't coming down to listen like a pervert. I am very glad my parents are still fond of each other but I experienced just a tad too much of this fondness. I know realize that I could of walked back up and try not to squeak on the stairs, but come on. I wasn't really thinking at that point, mostly freaking out! Thanks everyone and no I didn't have my camera to save the picture as blackmail ;)

Looks like their marriage is solid!

I think they're trying to tell you something