Saying that Greece was more "sexually open-minded" than Rome is a very presentist statement, using modern morality to view the past. While Athens in 330 BCE may have tolerated gay sex to a greater degree than the Western Romans in 400 CE, this view of homosexuality was very limiting and misogynistic by modern standards.

And very few historians subscribe to a "dark ages" model of the Early Middle Ages. That's a Western-centric viewpoint based on outdated Italian philosophy. Alan Moore should stick to writing comic books about prostitutes and complaining about Hollywood.

I read at least 20 different theories on the fall of the western Roman Empire, and sexually repressive society causing the breakdown of the social order isn't one of them.

Nor do I see a connection between the Greek attitude towards sex and the works of their philosophers and artists which is what we admire the most when we think of Greek civilization.

Speaking of which, a lot of what the Ancient Greeks would thought civilized we would consider abhorrent or just bad - massive xenophobia, attitude towards women, animal sacrifice, constant bloodshed, slavery. I don't want to diminish their contribution to European civilization, but lets not put them on a pedestal.

The guy who said this is not the first or the last person to seek historical justifications for his beliefs, or to mix up correlation and causation.

There are three main problems that I can see. The first, as others point out, is that Moore does not show any mindfulness to the fact that sexual "openness" and "repressiveness" are not objective or unchanging categories of meaning. This kind of anachronistic thinking is what led to tge transformation of the history of ideas in the 1960s towards forms of analysis more sensitive to historical context. To paraphrase JCD Clark -- if you find yourself congratulating the past for resembling the present, or if that is the only manner in which you can understand the past, you're probably doing it wrong.

The second is the idea that sexual habits in a culture (all of them? some of them? which ones?) underpin or have some kind of mechanical / deterministic effect on the course of history. This kind of monocausal thinking is seductive but almost always wrong.

The third is that, it's very long list of things Roman culture has passed on to the West that has contributed to its resilience and longevity. For one thing, I would put Roman law near the top for its role in shaping the various systems of common law, among the most distinctive and "civilizing" features of the Western way of life.