When we look back at the ancient world, it is very important to remember that it is an era that existed long before our present Christian era, with all its moral hang-ups and prudery. Alexander, Hephaestion and Bagoas were men of their times, not ours. As for Bagoas being a "boy," it is my understanding that he was a "youth" when he met Alexander, not a child.
Someone's already pointed this out, but Bagoas was 16 at the time of the book, a teenager, but still considered a boy. In the book, Bagoas himself uses the term for lover and beloved. The Beloved being the 'boy'. When he first sees Hephaestion and Alexander interact, he's surprised Alexander is already someone's 'boy'.
I can find no evidence that Alexander had any relationships with "boys." In this, he was proudly Macedonian, not Athenian!
Well, so was Phillip and he was famous for his relationships with boys. I forgot where I read about Alexander and his favorite page. The writer was speaking of Alexander's boy comparing unfavorably with Hephaestion. It may come from a legitimate source or from pseudo-Callisthenes, I don't remember. I'll have to paw through every source material I have on Alexander to find it again. I went through a huge Alexander phase when the movie came out, and I read a bunch of books on him and even went so far as to order this professor's dissertation on Hephaestion.
Alexander was the King. He did it "because he could." It's that simple! Hephaestion would have known and understood this, and loved Alexander no less because of it.
Ah, it's good to be king. Certainly Hephaestion understood it and probably didn't love Alexander any less for it, but that doesn't mean he liked it.