Xenophon, Constitution of the Lacedaemonians 1.2-10. (4th cent. B.C.)
It was not by imitating the customs of other states, but by knowingly doing the opposite to most of them, that Lycurgus made his fatherland pre-eminently successful.
(1.3) To begin at the beginning, here is his legislation about the procreation of children. Other people raise the girls who will bear the children and who are supposed to have a good upbringing with the most limited portions of food and the smallest possible amount of delicacies. They make sure they abstain from wine completely or give it to them mixed with water.
The other Greeks think that girls ought to sit in isolation doing wool work, leading a sedentary existence like many craftsmen. How could they expect that girls raised in this way could produce significant offspring? (1.4) By contrast, Lycurgus thought that slave women could make a sufficient quantity of clothing.
But as far as free women were concerned, because he thought childbearing was their most important function, he decreed that the female sex ought to take bodily exercise no less than the male. He established competitions of running and of strength for women with one another, just as he did for the men, because he thought that stronger offspring would be born if both parents were strong.
(1.5) As for a wife’s sexual relations with her husband, Lycurgus saw that men in other cultures during the first part of the time had unlimited intercourse with their wives, but he knew that the opposite was right. He made it a disgrace for the husband to be seen approaching or leaving his wife. As a result it was inevitable that their desire for intercourse increased, and that as a result the offspring (if there were any) that were born were stronger than if the couple were tired of each other.
(1.6) In addition, he stopped men from taking a wife whenever they chose and decreed that they marry when they were in their prime, because he thought that this was better for their offspring. (1.7) He saw that in cases where it happened that an old man had a young wife, the men were particularly protective of their wives, and he knew that the opposite was right. He required that the older man bring in a man whose body and mind he admired and have him beget the children. (1.8) But in case a man did not want to cohabit with his wife, but wanted worthy children, he made a law that he could beget children from a woman who was noble and had borne good children, if he could persuade her husband.  (1.9) He agreed to allow many such arrangements, for the wives who wanted to have two households and husbands who wanted to acquire brothers for their children, who had blood and powers in common, but did not inherit their property. 
Thus Lycurgus had different ideas about the begetting of children, and anyone who wishes to may judge whether or not he succeeded in producing in Sparta men who were superior in height and strength from the men in other states!
17. Note that while husbands can decide what to do with their wives, wives do not have a choice about what to do with themselves or their husbands; as in the Republic, they are regarded in Lycurgus’ legislation as vehicles for the production of children.
18. Cf. Hodkinson in Powell, 1988, 90: ‘Both monogamy and polyandry can be interpreted as practices designed to limit the number of legitimate offspring a man sired and hence the division of the inheritance’.