Pon farr

Pon farr is a physiological condition in adult Vulcans that causes them to require sex every seven years; if they don't mate, they will die. Vulcans can have sex at any time, but during pon farr, they must have sex. Sort of like going into heat, plus.

A clever conceit that first showed up on the original series (in the episode "Amok Time" by Theodore Sturgeon) and was gladly appropriated by numerous Kirk/Spock slashers.