I read campaigns of Alexander by Arrian who was a 3rd century Roman general. The book didn't talk at all about Alexander's sexuality. It mostly talked in detail about the nature of his war tactics and fighting and how he was never quite able to overcome the split between Persian (arab) and Helenic (greek) culture. I think that would have made powerful subject matter for the movie if they focused on that. To a Roman general being gay wasn't important. If he were a modern day general he would think about it in the way that we think about someone liking watching old westerns or something like that. It's just not that important, the focus on homosexuality says far more about our time than Alexander's.
The problem with the movie isn't the homosexuality, it's only showing two battles and Alexander whining and crying all the time and being a totally unsympathetic character (which is a completely separate issue from the homosexuality.)
This is another example where people basically try to squeeze reality to fit the axe they want to grind ("Americans reject movie with homosexuality") when it's really "Americans reject movie with horrible script."
There are basically 4 ancient ources for Alexander, all of which unfortunately lived a considerable amount of time after Alexander and based their histories on other primary sources which are now lost. It's one of the other 4 (I forget which) which implies Alexander's bisexuality. It's not like Stone invented the whole business recently.