agoge. The hardships of discipline are balanced against the beauty of life, so one doesn't indulge and make greed, lust and gluttony a part of their life. Some people would argue--and falsely--that the casting out of puny or deformed children was a sign that Spartans would support abortion; archaeology only reveals the dead bodies of adults, likely criminals, but that doesn't matter because art exists outside the realm of history (history is only a vehicle). There is a symbolic significance for the child being thrown out, and the child that is not thrown out (the deformed Ephialtes who betrays the Spartans).
If you examine the list of credits, there are two trans-gender characters, a contortionist and, of course, the two kissing females comprising Xerxes' entourage. Why is this important? For at least two reasons: first, that unnatural is very close to Xerxes and part of his inner-circle, which suggests that he himself is unnatural (yea, we knew that, this just substantiates it). Secondly, it reveals Xerxes' sexual perversions. These elements are strong, polarized characteristics (like black-white, male-female, right-wrong); when such ideals are incorporated into a story, the narrative conveys an entire package of cultural morals to the audience and justifies why society holds true the norms it has established (like not tolerating homosexuality, ambiguous sexuality and non-traditional marriage [Xerxes' harem and multiple partners as opposed to Leonides and Gorgo's traditional marriage]). 300, then, was not only identifying the enemy in Xerxes' behavior, but as well reminding viewers why we must adhere to the cultural norms we have always held as true: because once one sin is permitted to enter, more will follow, and it can't be stopped.