My son caught us having sex!!

I am mortified! My 15 year old just caught his stepdad and I having sex. They were supposed to be with their dad for the weekend. Our baby was asleep so we decided to have a go. We closed our bedroom door but didn’t lock it because no one else was supposed to be home. I don’t know what to tell me son. He stormed out and got into his sisters car. Any advice?