We know that it is a biology certainty that our parents once had sex, but that doesn't mean that we ever want to think about it. From your parents' flirty comments on each others' walls to dirty sexual jokes to (God Forbid) their "liking" one of your attractive friend's bikini pics, there is never a scenario where we want our parents' sexuality thrown in our faces. The ideal relationship between a parent and a child is a steady truce where both parties are allowed to imagine that the other engages in no sexual activity whatsoever. When you parents violate this unspoken pact, it is a familial act of war.