Constitution of the Lacedaimonians (1.7-1.8)

It might happen, however, that an old man had a young wife; and he observed that old men keep a very jealous watch over their young wives. To meet these cases he instituted an entirely different system by requiring the elderly husband to introduce into his house some man whose physical and moral qualities he admired, in order to beget children.

On the other hand, in case a man did not want to cohabit with his wife and nevertheless desired children of whom he could be proud, he made it lawful for him to choose a woman who was the mother of a fine family and of high birth, and if he obtained her husband’s consent, to make her the mother of his children.