I think there is pretty good evidence to say Spartans were by no means avocates of the gay lifestyle. History is actually a pretty funny subject. Let's say you and a group of friends go to a party or a concert, the very next day each and every one of you will have a variations in your stories. Two people get in a fight, each will have very different stories to tell just half an hour later. You ask them for details on that fight years later, if it was a significant fight and they still remember it, then they are likely to have stories that vary even more. I don't think anyone can disagree with this, yet we think we can figure out exactly what took place during a particular battle or during one event in history. We can get a pretty good approximation I suppose, but if we think we can figure out the whole truth, then we are fooling ourselves. Spartans lived over 2500 years ago, I think it is safe to say we will never know exactly what they were really like. Every country needs its heros and legends, Leonidas is one of Greece's great heros (they even have the words Malon Labe meaning "Come and get them" as their military slogan, referencing Leonidas telling the Persians to "come and get them" when they asked for them to hand over their spears). Let them have him, and let them depict him they way they have traditionally. Modern Greeks are extremely homophobic, and Frank Miller wrote the graphic novel with a very strong bias in favor of the Greeks.

EDIT: My Greek Characters for "Malon Labe" didn't work, so I have to transliterate it.