Are the Ancient Greeks testament to the true nature of the fluidity of uality?

Ancient Greek writers such as Plato and Plutarch would have us believe that relations in Ancient Greece - often occurring in the context of a young man's education (paideia), between citizens and slaves, and often encouraged in military contexts such as in Ancient Sparta or the Sacred Band of Thebes - were common. I have little doubt that ual orientation among the Ancients would have been no more prevalent than it is today (that is, of course, if we agree that ity is a product of biology), yet ual relations are portrayed as seemingly rampant. So I ask this, were the ancient Greeks testament to the fluidity of uality in such a way that they demonstrated that man is able to form relations with either to an extent much greater than it is commonly believed today? I don't wish to offend anybody, but do you think that the history of ity in Ancient Greece puts to question the veracity of statements such as "I couldn't possibly have with him/her, because I'm straight/gay" or even the notion that straight men are only able to form close emotional bonds with women, and that gay men are only able to form close emotional bonds with men?