Robert Silverberg - The World Inside

Earth 2381: The hordes of humanity have withdrawn into isolated 1000-story Urbmons, comfortably controlled multicity-buildings which perpetuate an open culture of free sex and unrestricted population growth. Nearly all of Earth's 75 billion live in the hundreds of monolithic structures scattered across the globe, with the exception of the small agricultural communes that supply the Urbmons with food. When a restless Urbmon computer engineer begins to think unblessworthy thoughts of making a trip outside, he risks being labeled a flippo, for whom there is only one punishment.