I wouldn't say I immediately regretted it - not until after...
My best friend of a number of years, we had been living in a house together with another guy - we all lived together and split rent. Oh those teenage years. Well, my buddy had a really attractive girlfriend that at first I didn't even pay attention to. But the thing is, my buddy works a lot.
So we would be partying and stuff and she would just be hanging out alone, we were friends before they started dating so she felt comfortable to kick it with me. Well, over the course of the next few weeks things kept getting more and more... complicated.
I thought about telling my buddy, but at the same time I didn't want to put her on the spot. There was multiple self talks to myself about not sleeping with her, but each time we hung out I could feel it getting closer and closer to that end.
One night we, my buddy, his girlfriend, and I - got drunk. The entire night his girlfriend and I exchanging glances, looks. I tried to play it off, but odds are it was probably pretty obvious.
During one of those looks my buddy got called into work which left just us two in the house. The minute he left... it was on. It was like everything up until that point was just foreplay. I took her right there on the table.
Not even 10 minutes later, probably considerably less, I finished and pulled out. It was then it hit me just how bad this was, I had just fucked my best friends girlfriend - I most likely just ruined years of friendship over a piece of ass. Fuck.
And as if this realization wasn't damning enough, my buddy came walking back inside - my still erect penis just inches from his girlfriends cream-pied vagina... I could not have been more caught.
I was ready for him to come at me, anything but what he did - but instead he just said "I knew it. I fucking knew it." then he just slammed the door and left.
That was the end of our friendship, and rightfully so. I regretted it the instant I was done - it wasn't worth it, not even close. It wouldn't have been worth if it I had even got away with it- because I knew every time I saw that guy after this... my shame is all I would ever think about.
TL:DR Had sex with my best friends girlfriend, got caught immediately after. Ruined our friendship.