Coolidge Effect & Habituation
Definition: In biology and psychology, the term Coolidge effect describes a phenomenon—seen in nearly every mammalian species in which it has been tested—whereby both males and females exhibit continuous high sexual performance given the introduction of new receptive partners.
Animals tire of intercourse with their present partner and get excited at the prospect of a new sexual partner. The reason: the amount of dopamine declines with current mate, but shoots up with a new mate. This program for novelty is what makes Internet porn so enticing to your primitive brain.