מדרש על סנהדרין 216:15

After the sacrifice was completed, God blessed Noah and his sons. He made them to be rulers of the world as Adam had been, and He gave them a command, saying, "Be fruitful and multiply upon the earth," for during their sojourn in the ark, the two sexes, of men and animals alike, had lived apart from each other, because while a public calamity rages continence is becoming even to those who are left unscathed. This law of conduct had been violated by none in the ark except by Ham, by the dog, and by the raven. They all received a punishment. Ham's was that his descendants were men of dark-hued skin.