Sex Drive is the biological drive to kisspop, usually experienced by creatures at the adolescent life stage or later, which can result in a pregnancy.
The typical system where increasing a drive gives Punishment is thought to discourage kisspopping in Creatures 1, so this is often gengineered. A common change is to give a small Reward for increasing sex drive, but a larger Reward for lowering it.
In Creatures 1, although creatures could learn a word to express that they had a high sex drive (the Encyclopedia Nornica teaches them to say 'amorous'), any attempts to say this are thought to have been blanked out by the game engine, resulting in a blank speech bubble. In Creatures 2/Creatures 3/Docking Station, the word used to express having a high sex drive is "friendly".