I personally don't believe the Theban Sacred Band was made of homosexual lovers. If gay lovers were such great warriors, wouldn't every city-state create a band of such soldiers?
IIRC Polybius is the primary source on this, although he was writing after the event, it was still recent enough for the tradition to be widely known. Don't forget even the Spartans institutionalised pederasty within the agoge, so the practice was hardly unusual amongst Greek fighters. The Thebans may well have recognised long-term partnerships between men, but a Spartan, pederast though he may be, was still expected to produce more little Spartans.

I don't think it's that much of a coincidence that the major colonial powers in Greece (ie those suffering population pressure) would be more willing to accept long-term male partnerships than those with a chronic manpower shortage such as Sparta.