Dopamine plays a major role in motivation, including sexual motivation. For more see Dopamine. Also see Coolidge effect (habituation).

Nucleus accumbens dopamine increases sexual motivation in sexually satiated male rats

PPL2ab neurons restore sexual responses in aged Drosophila males through dopamine

Fruit fly sex drive hints at how animals choose behaviors and “Dopaminergic Circuitry Underlying Mating Drive” (Lay)

Yohimbine interacts with the dopaminergic system to reverse sexual satiation: further evidence for a role of sexual motivation in sexual exhaustion

Induction of mating behavior by apomorphine in sexually sated rats