Have you not been taught about biology? All children should be. To reproduce, animals, including humans, mate. They combine genetic material to produce offspring.

Sex, the physical act of mating, is accompanied by a pleasure response, so it is performed for recreation as well as to reproduce. People often attach emotional significance to both the act and the pleasure. The inappropriate books in the library are explicit, sensationalized accounts and depictions of the mating act.

[OOC: I hope Sephiroth's sex-ed class is okay; I tried to keep it age-appropriate.]