I can identify with where you were at. Sometimes our lust and desire take over, and things happen in the heat of the moment.
The first time I took a cock up my ass was also the most painful and, at the same time, one of the most satisfying. We had been out drinking with a friend and decided to grab a cab at closing time. My wife and the friend had chemistry, so we asked if he wanted to return to our house with us instead of his apartment. I quickly got my wife undressed, and as the friend watched, I got inside her and was going hard in a missionary position with her underneath me. The friend got undressed and asked about lube so we could give my wife DP. He got the lube out and began to spread it while I was still on top, so my ass was getting most of the lube.
The friend was fumbling around, trying to figure out how to join in with our position, and then suddenly, and I felt an incredible bolt of lightning rip through my body. Still, inside my wife beneath me, I screamed in agony, and my entire body became stiff. My wife asked if I was cumming, and I could not speak. I realized our friend had his cock up my ass, and he was fucking me. I was trying to tell him he was in the wrong hole, but with tears streaming down my face, I couldn't get my words out. His body weight was pushing me deep into my wife, and she was starting to climax, and I began to enjoy what I was feeling.
It was not until all three of us had climaxed and were lying in the bed that our friend realized he was inside my ass instead of my wife's. That was fourteen years ago, and we still all three laugh about it.