Musar על סנהדרין 213:4

Marital relations are completely permitted but the sages decreed immersion in a Mikveh for those who had seminal emissions in order that Torah scholars not be frequently with their wives like roosters. For even though the conjugal act itself is permitted nevertheless he imprints this lust within his nature, and from there he can be drawn to the forbidden as our sages said: "there is a small organ in man. If one satiates it, it becomes hungry. But if he starves it, it becomes satiated" (Sanhedrin 107a).