Appropriately enough for Romance, it all comes down to Rome.
Nyrath the Nearly Wise, author of the invaluable Atomic Rockets website, noted in a comment on my last post, Ken McLeod called history "the trade secret of science fiction." As a heavy consumer of fake history, SF has developed an efficient manufacturing process for churning the stuff out. This takes the form of a broad consensus version of the future, first identified as such by SF critic Donald Wollheim. You can read my own sketch of it in the Future History entry of my "Tough Guide to the Known Galaxy" (title and concept stolen, of course, from Diana Wynne Jones).
In a nutshell, humans go into space in a big way, and sometime in the next few centuries (usually) they spread out to the stars. The new worlds they settle are presently united in an Empire - but sooner or later the Empire falls, plunging the worlds into a new Dark Ages of backwards, impoverished isolation.
In the future, it seems, time on some scales runs backwards. The first few centuries of this future history are cribbed from the colonial history of North America: Columbus and the Pilgrims and the Wild West all jumbled together, but culminating - at least in US science fiction - in a new Revolution, the free and noble colonists throwing off the heavy hand of
King Geo of Earth. At a later period, however, Rome becomes the governing theme: grandeur and decadence, the fall, and the Interregnum. (Science fiction has its own technical term for a Dark Age; how cool is that?)
As a dramatic stage this future works pretty well, which is why it has become the standard - an author can allude to it in a few words, and experienced SF readers will recognize the framework. It has the further benefit of being rather plausible, so long as you grant its pretty dubious premises, such as cheap and convenient interstellar travel. Some nice tensions are built into it as well. We cheer the freedom-loving colonists (don't we?), yet the fall of the Empire is grand tragedy.
Far-called, our navies melt away;
On dune and headland sinks the fire:
Lo, all our pomp of yesterday
Is one with Nineveh and Tyre!
This theme came into SF with Asimov's Foundation Trilogy, a book that had enormous influence on me when I read it in junior high. (This is possibly not a good sign; it is one of the very few things I have in common with Newt Gingrich, and a persisting buzz surrounds the fact that the Arabic word for "Foundation" is al-Qaeda.)
Asimov got it of course from Edward Gibbon, who not only wrote The Decline and Fall of the Roman Empire but largely created it. That the Western Empire broke up into fragments was not exactly news, but Gibbon taught us to see it the way we still do, rude barbarian encampments under the broken aqueducts.
Gibbon had not a good word to say about the Eastern Roman Empire, even though it outlasted the western half by a thousand years. Later historians improved on him by renaming it Byzantine, lest the Roman name be sullied by contact with the effete eunuchs of Constantinople. What a contemptable people those Byzantines were, arguing abstruse theology in taverns when they weren't smashing statues or blinding each other, or even negotiating with Muslims; achieving nothing for a thousand years but staving off Visigoths, Avars, Slavs, Persians, Arabs, more Arabs, Bulgars, Vikings, more Vikings, Turks, Normans, more Turks, and still more Turks. They failed to stave off the Fourth Crusade, and finally there came more Turks than they could handle.
Byzantium is nowhere in science fiction (anyway, most of us 'Murricans never heard of the place), but it shines glorious in fantasy, disguised as Gondor. This is Byzantium roughly as seen by a tenth-century Englishman, who has not yet learned to sneer at it - "English and Danes" were still among the stoutest defenders of Constantinople in 1204, when the Fourth Crusade showed up.
Rome, too, shines in fantasy, though refracted almost out of recognition. The whole broad genre of Romance begins with Arthur. Whatever the obscurities of the "historical Arthur" - where the claim can be made with a straight face (I'm rather sympathetic to it) that there really was an Arthur, but he had a different name - the original one brief shining moment came in Britain, sometime around AD 500, when for a little while it seemed that that a people who still sensed their romanitas might hold their own against the tide.