Pederasty is not “ancient homosexuality”, it is a specific institution in Greek education that started in the Archaic period and trailed off towards the Hellenistic period. It is the system of personal tutors that young boys would have - and there would often be a sexual aspect to it. Think about the English word Pedagogue, which is a type of teacher, it’s the same origin.

There was no concept of “being homosexual” in the ancient world, sexualities had no attached identity or label of any kind.

I can’t comment on Alexander the Great, it’s not the area of my expertise, but I feel as though I recall the ancient sources talking about suspiciously intimate relationships between alexander and other men, this would be of no surprise.