They also practiced eugenics in an effort to breed a race of super warriors. In short, they were proto-Nazis.
Still, there is something about a last stand against overwhelming odds that still fires the imagination.
The point of the movie was not that we should emulate Spartan society or politics. There were many unattractive things about them.
However, the Spartans did have the courage to stand up against overwhelming odds to protect their society from an aggressive ME society that wished to enslave them. They lost that battle, but subsequent battles were won and the Spartan/Greek world was saved. That was the message, and I think that's what most people got from it.
Historical Fact:
Infanticide:
Infanticide has pervaded almost every society of mankind from the Golden Age of Greece to the splendor of the Persian Empire. ...infanticide has been for centuries a prominent and socially acceptable event in two related areas of the world: India and China.
http://www.infanticide.org/history.htm
It has long been accepted that the practice of exposure (expositio) existed in the Roman world, and is considered to be the main way in which the Romans asserted control over the size of their families. The practice was considered to be normal with parallels existing in Roman mythology, such as the story of Romulus and Remus.
http://www.cf.ac.uk/hisar/teach/ancthist/projects/children/expo1.html
Infanticide existed in most ancient civilizations, from Greek, to Roman, to Persian, Jewish, Chinese, Indian, Japanese, Korean, Egyptian, Carthage, Eskimos, etc. and it did not get banned until major religions such as Judaism, Christianity, Islam etc. came into play.