Sex is a base instinct.
It has always been and always will be. Even in sexually liberated Rome sex was the bread and butter of perverts and miscreants. Take a look at the walls of Roman and Greek brothels and the perverse nature of their orgies and festivals.
Sex isn't viewed of as naughty and perverted because of moral people, but in spite of them. They try to show sex as something beautiful and special shared between two people in a loving relationship. It's those on the left who bring sex into any and everything.