My wife and I were both very close to my best friend for years. Either subconsciously or because I’m an idiot, I never really copped on to the hidden tension/attraction between them. But over the years it grew and almost inevitably, one night, almost organically, we had a threesome. That was my first time seeing him naked. Turns out he was hung! Very hung. The threesome was amazing, but my favourite part was when I was watching rather than doing anything. And the way she reacted to the bigger cock was mind blowing. I thought I was good in bed, but I never made her cum from penetration alone, he did. Watching how enthusiastic she was, almost cock hungry, I as such a turn on.
We did it one more time and then stopped. I have several regrets from this experience, all around how looking back, I wanted soooo much more! I think about it often and I fantasise about how it could have developed, gone further. But my wife got cold feet, I think because she enjoyed it too much and that made her nervous.