Dad's new wife
My parents divorced when I was young, and eventually my dad ended up remarrying. Eventually it came out that she was the woman he'd cheated on my mom with, so I never liked her much. Their relationship ended up being incredibly toxic, but man did they fuck like rabbits.
My bedroom and theirs shared a wall, and their bed was against the wall we shared so I could always tell exactly when they were fucking. The headboard would knock against the wall with every thrust, and I could hear his wife moaning. It made me uncomfortable at first, but the more it happened the more I found myself wishing I was the one getting fucked like that.
Eventually I started touching myself whenever they'd have sex, imagining I was the one getting plowed into the mattress by my father instead of his bitch of a wife. I'd match my movements to what I could hear on the other side of the wall, trying to make it as accurate as possible.
My door's latch didn't ever close all the way and once the cat pushed their door open a crack while they were fucking and I could see them from my bed just a tiny bit. I swear my dad saw me watching them, but even if he did, he didn't stop. If anything he started fucking her harder. I never made myself cum as hard as I did listening to my dad fuck my stepmom and imagining he was fucking me instead