Overhearing straight couples fucking is, no joke, one of the things that turns me on the most in the world.
When I was a college sophomore, I moved into a shitty duplex-type apartment with EXTREMELY thin walls and the straight couple next door were VERY vocal. Their bedroom (and their bed) was literally right next to mine, only separated by a paper thin wall, so not only could I hear everything but I could also feel the vibrations in my bed when they were fucking hard. The woman was a screamer, she had the loudest orgasms I've ever heard, and super filthy too. She would be like, "Yeah, daddy, spit in my mouth, spit on my pussy, your big cock feels so good, I want your balls in my mouth," etc.
Well, one day I discovered that I could see straight into their bed from a little grimy window on the little narrow alley on the back. That night I hid in the shadows and jerked off watching them. Her ass was huge and what seemed like inches away from my face as she was riding his dick. Neither one had very attractive faces but their bodies were absolutely on point.
Anyway, over time we became friendly and one thing led to another and they ended up inviting me to a threesome, my first threesome ever. She wanted to get DP, so she rode my dick with me laying down as the dude plowed her ass. My god, the way that woman orgasmed, the full-body shakes and the screams and moans were next level.
The couple moved away a few months after that but to this day I still remember that as probably the horniest time of my life lmao.