My mom knew when I lost it...actually thought I lost it before I really did. I had to convince her that I hadn't had sex because she was ready to put me in a chastity belt.
I never talked to my mom much about it. However, recently we've moved relationship status where she's more a friend than a mom...weird. We talked a lot about things that have gone on in the past year. She doesn't know everything, but knows more than most moms do I think.
I don't think sex should be tabood. I also think parents know a lot more than they let on...