Priapus

Zeus, King of the Gods, has grown tired of mankind's tendency for destruction and hate. Though he wishes for the swift end of humanity, he is unwilling to stoop to man's level by employing violence to achieve this particular goal. It is with this in mind that Zeus orders the god of fertility and reproduction, Priapus A. Tarou, to carry out the task.

Priapus comes up with a simple yet effective plan: If he were to turn every man on Earth gay, human reproduction rates would fall dramatically; in a few generations the human race would be rendered extinct. With irresistible charm, a collection of sensual god tools, and vast sexual prowess, Priapus begins his quest of conversion, educating mortal men about a new kind of intimacy.

(Source: MAL Rewrite)