I think a lot of this is related to how we are raised by our parents. Most parents aren’t over the top affectionate with each other in front of the kids. A hug/small kiss is normal, but they don’t make out in front of their kids do they? Kids grow up thinking that the relationship between the parents is the same as the relationship between the parents and children. When you spend most of your childhood life thinking one thing, it is hard to get used to a totally different aspect of their relationship. In addition most parents also hide the sex from their kids (you don’t have your dad saying stuff like “the other day when me and your mom were making love, the FUNNIEST thing happened….”). They are as embarrassed about it as the kid would be. The whole situation is a form of social conditioning really.
@ elcho and rockstar – i would be mortified if i ever walked in on my parents doing the deed. luckily it has never happened.