Hell, our son is aware that we have an active sex life. He's 16 and he occasionally teases his dad and I that it is a good thing he has an iPod with headphones.
I think it is important for him to know that a healthy and happy marriage includes sharing an active sex life together. I hope when he gets married, that knowledge will carry over into his marriage.
We have sex every day. We live here and our kid lives with us. Sex is as normal, as eating in our home. We do it, it happens, it is normal and our son knows it is too. Granted, it may make him cringe, (hehe) but that is OK too.