Did you notice what went wrong in that scene?
Ordinarily, Billy would work hard to make money from his paper route. Then he'd use the money to buy dinner for Mavis, thus earning the slim chance to perform the reproductive act.
But in a world where teens can look at cartoons, why should he bother? Why should anyone bother?
Let's take a look at Billy's planet a year later: Where are all the football stars? And where are the biochemists? They're trapped! Trapped in a soft, vice-like grip of animated lips.
All civilization was just an effort to impress the opposite sex ... and sometimes the same sex!