It happened by accident
Back when I was a senior in college, I lived with a guy that I'd known for many years, although we were never best friends. We'd lived with a group of guys in a house for several years, but our final year, we shared a 2-bedroom off campus. We both played hockey on a club team on Friday nights, and then afterwards we'd hit the bars and usually hook up with chicks and go our separate ways. The following Saturdays, we'd eventually run into each other and share the story of our conquest the night before.
One particular Saturday morning after an unsuccessful hunt for some action the night before, I wandered out into the kitchen in my boxers and a t-shirt to make some coffee. The smell of coffee woke up my roommate and he wandered out into the kitchen, similarly dressed. Both of us had struck out and were a little punchy from staying out late. We started razzing each other and giving each other a hard time. That turned into some arm puching and horsing around. That turned into a all-out wrestling match that moved from the kitchen into the living room. We were both in very good shape, so the apartment usually took the brunt of these brawls.
I finally pinned him and told him to admit he was a wuss. He wouldn't and called me a pussy. I said 'who's the pussy? You're on the bottom!' I started to fake-hump him and said, 'You're the pussy!' He was trying to throw me off of him, but I kept him pinned. But then I noticed that he was getting hard. So I ground into him again. Sure enough, he had a ranging hard-on in his boxers. Then I got hard. It took us by suprise, but the urge got to us and we started grinding our bulges togther until we were really horned up.
I got up, kneeling, straddling his thighs. He reached down and pulled his cock out of the fly of his boxers, and then I did the same. We started to stroke our own cocks. I was staring at the ceiling, really getting into it when I felt his hand on mine. I looked down and I let go of my dick. He then took over and stroked my shaft. I decided to help him out and I grabbed his, too. We were both precumming and fucking each others' fists until the build up came and we both shot loads on his abs and chest.
Afterwards, we went into our respective bedrooms and cleaned up. We never talked about it, and we never did it again.