I hope you can forgive bad formulations and mistakes, English is not my native language.
A little information about the situation: In my country army service is mandatory for a time. I have this buddy, Karl, was in the service with. I became a professional soldier/ lifer after our mandatory service ended. He did another voluntary term and then quit the service.
My wife, Clarissa and I, both dabble in partner swapping, same as Karl and his longtime girlfriend, Christine. Seeing as we were old friends we always were hesitant and avoided to actually do anything in regards to swapping together. We didn’t want it to be weird between us or our SOs, or so I thought.
That all changed when Karl and I sparred. We both do martial arts. Usually we make small bets on the outcomes of our sparring matches. I am a competitive son of a bitch. Normally we bet small sums or dinner on our little matches, but I recently had my ass handed to me by someone else in training and my ego was hurt. So when I mopped the floor with him in the ring, I wanted to keep going. After another bout I won I pestered Karl again to continue. So to get me to back off he wanted to up the stakes. I asked him what he was suggesting. And he said a night with our respective SOs. I always have had the hots for Christine, his girlfriend. So I was sure of my victory, my ego was swelling at that point and I saw an opportunity, so I agreed.