I started fucking a friend's wife a few years ago, and though she has had a rich sexual history, and my cock is only slightly above average, I gave her her first orgasm ever the first time we fucked.
We continued fucking behind his back, and a week later she told me she was leaving him. She explained that it wasn't because of me, or for me, but I was really young and confused about how that could be true. Now I understand that she just needed someone to sort of cushion her fall when she did leave him, which she was probably contemplating for quite some time.
We lived together for a summer, and it was awesome. She taught me a lot about how desirable I am, and can be, and kicked off a short but extremely active career of womanizing after we split up and the end of the summer.
Saw her again recently, and we're both in relationships. We had nothing to talk about, nothing in common. We silently agreed not to see each other again I think.
To this day, I still feel like I contributed to breaking up my friend's marriage (though she had always been a closer friend than him) and it is the one thing I legitimately feel guilty about, I think.
But it was really exciting to know I was fucking another man's woman.