Many parents still find sex difficult to talk about with their children. Talking about sex requires parents to acknowledge that their children are nearing adulthood. Parents like to think of their children as perpetually young. Talking about sex is a big step in the maturation of parents as well as children.
If sex were a taboo topic there would not be parents. Parents would not have children if sex were a taboo topic. Obviously they participated in the act or you or I would not be here. I believe talking to parents directly about sex can sometimes be uncomfortable, but that doesn't make it taboo.