We are asking, from the point of view of modern culture and ideas about sexuality, about a culture 3000 years in the past and their ideas about sexuality, and we want their ideas translated into modern ideas. "Gay" is a word we use now to describe certain types of male homosexuality, right? Well, what is male homosexuality? Is it the case that if a male person has one sexual experience, sometime in his life, with another male, he is homosexual? What if he has two... three... what would you take as a dividing line? What if a male desires other males, but not usually as much as he desires females, and never has a sexual encounter with another male; is he homosexual? What if he desires other males more than females, but never has a sexual encounter with another male? What if he desires other males less than females, but lives in a culture in which male-male sex is preferred, and has that kind of sex; is he homosexual? You can create a few more combinations here and puzzle over them if you want.
The latter case was, as far as we know, more-or-less the case in ancient Athens. Male/male sex was considered preferable to male/female sex as being an encounter between equals, and sex between an older man and a younger man was the most preferred, for a variety of reasons. Were the ancient Greeks homosexuals? From what we see in the Dialogues, Socrates actually seems, relatively, pretty "hetero", in that in at least one or two cases he refused offers of sex with other men. But there's no indication that he always refused it. He was married and had children, but that was the obligation they all had, otherwise the state would disappear.
As far as Aristotle goes, he and Plato probably had a lot of sex with men... were they "gay"? "Homosexual"? By their standards, our terms would have made no sense. Their culture preferred the opposite of what our culture prefers; how do you compare them, then? If you're evaluating it in terms of personal preferences, we have no idea at all of those; but we do know that one's preferences are due to some degree on one's culture and upbringing... but not entirely... so we're back to ground zero in terms of saying what, sexually, Plato and Aristotle, for example, "were" by modern standards. They were almost certainly men who had sex primarily with other men and probably preferred it that way, for some reasons quite dissimilar to, and probably other reasons quite similar to, the reasons men today have sex with other men.