reject homosexuality? The Spartans encouraged it between soldiers.
I think that poster is confused, because the comic book author puts in the mouth of Leonidas an insult directed at other Greeks concerning their homosexual practices. It's just another reason not to treat the comic or the film as real history.
The classical sources are all over on the question of Spartan homosexuality - some of the main sources suggest Sparta actually discouraged the sexual aspect of the relationship between older men and adolescents, focusing on the mentoring and friendship roles between the two. Other sources suggest they were as bad buggerers as the Thebans and others. Who really knows? I suspect there is some homoerotic element in most mentoring relationships, whether conscious or not. The whole issue is not whether the element exists, but whether the mentor and mentored keep things on a platonic level or not.