This is a fucking hot question. Hot because it is a mindfuck. Mindfuck because it is true.
I thought I was ok at sex...I thought it was a bit of an exaggeration. But the moment they started to play....I have never experienced such a fall from grace before. I was literally ashamed of myseld. The guy kissed better, held her better, got deeper inside her, was way more intence, and, most of all, had the endurance that I had never seen. My gf at the time (who left me for him) had never had three orgasms in one session with me. I don't even know if she had one. With this guy, she kept cumming on his cock. From that point on, my confidence level went into the toilet, and it is still there. Now, I don't fuck my current girlfriend at all. SHe only goes to her bull.
In a nutshell, it had a DRAMATIC EFFECT on me.