I've slept over and banged out chicks with teen sons, who were probably aware that mom was getting dicked down, but never with a grown son in the house. It's sticky getting into family mess, because in the end, they'll stick together. You did the right thing getting the hell out of there, and doing what you could to avoid escalating things.
"The best kind of pride is that which compels a man to do his best when no one is watching."