Basically, started out as whatever you want it to be; just don’t do it so much you get all spun out from. But with the Socratic method of humanism by repeatedly updating inquiry, filtered through philosophers you get a more complex ethics in dealing with it. In essence, anything that leads one to renounce agency through non-rationality became viewed as suspect (which is why he went from the post-industrial ‘Enlightenment’ era to Greek antiquity in volume II). But he doesn’t make this the central thesis, arguing instead inductively through the actual texts most centering around political, practical, economic—society’s—concerns in order to make a wider case for the Greek world (really Athenian, but representative) as a whole; so the result is suggestive of an ethical complication in Greek texts which is over-emphasized in the work as stated. I guess the argument at its core is that things were already problematic regarding sexual acts to begin and this developped into anxiety concerning sexuality abstractly—as per Platonic philosophy—in relation to the history of subjectivity.
So the supposed Christian relegation of sex to immorality would, at the most basic level of argument, derive from a problematic relation to sexuality—possibly human subjectivity and/or its agency—at the core of western philosophy (appearing as theoretical components of religion, ideology sought to contain what it could not rationalize): It’s a political question of ethics in determining what qualitative amount of ‘sex-is-being’ one wishes to comport into prescriptive attitudes based on what constitutes a “good” life as relation to what kind of pleasure.
Either way, by Augustine’s time the debate was settled; it’s just that no one had written articulately and personal enough to demonstrate.