Do parents generally know what kind of sexual behavior their kids are involved in?
Most of the time it turns out that parents already suspect if their kids are sexually active. Parents notice things. They notice stains on underwear, for example. But a lot of parents don't know how to raise the subject. The best time to talk about when it's right to have sex, I think, is when a child is in the early teen years. Pre-teens think sex is yucky. Some kids start having sex in their mid-teen years. If parents haven't given their children guidance by then, it may be too late to impact behavior.
Personally, I think parents need to send two clear messages to their kids. First, they need to tell them when, in their opinion, it's appropriate for a young person to have sex. Second, if their teenager does decide to have sex, I think it's vital that parents express how important it is to protect themselves, and their partners, from pregnancy, sexually transmitted infections, and emotional hurt.
But some parents are just very uncomfortable talking with their kids about sexuality. I had a mother bring her daughter in for a physical exam. As I was going into the room to see her daughter she handed me a note that said, "Please get Mary on the pill."
Can you predict which parents will have a tough time talking about sex with their kids?
I think that parents' communication with their kids about sexuality to a considerable extent reflects their larger relationship with their kids.