I am a Single mom have been for a few years well i have two kids a 17 year old son and a 10 year old daughter. Well my son has been with his girlfriend for the better part of 4 years a long time for teenagers and proud of the both of them. Well during the summer and school year i have always trusted my son to watch his little sister. Because i tend to work till about 5-7Pm give or take. Well the other night while he was supposed to be watching his sister i got home earlier then i thought and my daughter was watching TV which is pretty normal so i went upstairs to find my son and walked in on him and his girlfriend having sex. I am not entirely sure what to do about this i have been thinking about telling her parents i haven't talked to my son yet about it yet either, other then grounding him for a while until i deal with this. What should i do? Any advice is welcome.