One thing that is beyond serious dispute about the era in question is that homosexuality didn't just speak its name, it shouted it from the rooftops. If Alexander wasn't having homosex then he was probably just about the only one..
Moreover, the Romans took a much more ambivalent view on the matter, and since they revered Alexander there is rather little reason I can think of why they would've fabricated such a thing.
I repeat.
Would you make broad statements about today's culture if all you had was a few books that had been copied and recopied by hand and the best copy was 500 years after the fact?
It's like this: the info we have about Alexander's greek culture should be compared to Dan Rather's memogate.