More specifically, Xenophon tells us of a practice known as “husband-doubling.”** By this arrangement, an older husband and a much younger wife could select a younger man to father children for them. Similarly, a man who wanted children could make an arrangement with a married woman to have children with her, provided her husband agreed.
There were several benefits for the individuals involved. There was the potential for strong sons, which brought prestige to all parents involved. It also, as Xenophon himself points out, gave the woman involved access to and power over another oikos besides that of her husband. In a situation where paternity might be uncertain, both the woman and her children reaped the benefits of having two men contribute to their welfare. Even where paternity was clear, it gave the children siblings on whom they might rely, but with whom they would not fight over the inheritance.
It is important to remember that the entire purpose of this was eugenics. The Spartans wanted the best, healthiest children they could get and were absolutely willing to control reproduction to get them. While certain people were encouraged to have children in a husband-doubling arrangement, others were doubtlessly discouraged from the same, or even encouraged to avoid reproducing with their spouse and let someone else procreate instead.
What other effects these arrangements might have had on Spartan society are unknown. Certainly it is claimed that the Spartans were bigger and stronger than other Greeks. Potential confusion over paternity and the availability of extra people to act as parents may have led to wider definitions of “mother” and “father.” They may also have drawn mythological parallels. The stories of both Helen and Leda lend themselves to such interpretation, but as no Spartan versions of these myths survive, we will likely never know.
*At least by the people in any position to influence politics or write something we still have 2.5 millennia later.
**It used to be called “wife-sharing” by scholars who took Xenophon’s lack of acknowledgement of women’s desires as a sign that only men had the right to make such decisions and ignored the evidence that Spartan women were known for speaking their minds. Such scholarship also tends to omit what Xenophon says about the benefits women gained from such arrangements. Pomeroy suggests “husband-doubling” as a more accurate term, especially considering that Spartan households consisted mostly of women, as the men lived most of their lives with their army groups. This combined with Spartan women’s outspokenness suggests that women most likely had a lot of decision-making power in situations like this.
Xenophon, Constitution of the Lacedaimonians 1.5-10 - Perseus
Pomeroy, Sarah B. Spartan Women. New York: Oxford University Press, 2002.