Accidental cheating…

Its my forst time writing, though i have been reading for quite some time. I thought it was time to tell my story.. the beginning part was later explained to me by my wife..

My wife and i have shared responsibilty of dropping my son off at school due to my rotating shifts at work, so we both get a chance to meet the other parents that drive their kids in. There is one woman we have both noticed as being very good looking, and coincidentally very similar looking to my wife.
Kat is approx 5’9″ long straight black hair, full 38D tits and a great body. My wife Jessica is the same height and same tit size, though my wife has not lost all of hher baby-weight from giving birth like Kat has. During the first few weeks of the second year we have been seeing Kat at the school, my wife decided to invite her over to our house for the first time. I had worked the night before and gotten home while my ife was at the school, so i had no idea that she would be bringing Kat over, and she had no idea i would be sleeping in only a pair of boxers.
When my wife and Kat came in the door my wife offered the tour of the house, quickly showing the main floor before moving upstairs. When they got upstairs my 2yr old daughter ran into the play room to play while my wife continued the tour. They quickly made it to the spare room where i had decided to fall asleep. When they walked in, they found me asleep on my back, on top of the covers, and apparently sporting a rather large erection in my sleep.