The point is not to emphasize "how wicked literature is" but to emphasize that for many Christians it is like eating meat offered to idols. In Paul's day, if you remember, the only place anyone who was not fabulously wealthy could regularly get meat was to visit a temple.

Now by curious chance some of those temples are now cathedral's. And Yule is now Christmas. And we CHRISTIAN a ship with champagne, rather then sacrificing a thrall. And likewise I have far less vain scruples then I once did. But the fact remains that much of our narrative tradition is subchristian and that provides difficulties. Even Tolkien needed a theistic justification for creating a pantheon.

I suppose it could. I am not sure what I am trying to say.

Lets put it this way. Sevenwaters Trilogy is one of the best fantasies ever written. However it is written assuming the truth of animism. Which is fine enough as that provides very good material. The problem is that to many Christian authors would not be able to stomach writing it and many Christians would not be able to stomach reading it especially when actual animism is becoming a real part of the Occidental World in some circles again. Though now that I think of it, that is nothing new; Yeats for instance literally believed in fey.

Be that as it may, many of the most useful tropes in literature would offend the scruples of many Christians both writers and readers and that is a problem.

For instance even Medievalistic Romances are often glorifications of adultery, and can only be accepted by a Christian through Willing Suspension,even though the time when they were writen was supposedly a "Christian Civilization" a thing that never actually existed(In real life there was only a pagan civilization that had Christians in it).

True, but in this case, the point is that much literature isn't just about sin as sin, but about delighting in things that would not exist except in the context of a sinful world even if not sinful themselves. That does not mean such things cannot be used lawfully; indeed it it may be impossible to write well without doing so. The point is that such things exist. Using pagan metaphors in literature isn't the same as idolatry; it is plundering the Egyptians. Nontheless the fact remains. The point is not that Christian literature is impossible, it is that non-christian literature has the advantage that a lot of literary conventions come from non-christian sources. Not necessarily sinful as such or completly sinful. Just not Christian.

C S Lewis made the same point in the Christianity and Culture essay series.

Much of the problem is that much of literature depends on subchristian(not necessarily sinful but subchristian) tropes, and Christianity fits ill in it.
Science fiction almost universally depends on Darwinism, and Fantasy on paganism because in both cases the myth is to powerful to be left alone by authors.

Other stuff follows similar lines. How do you write a wooden ships story that doesn't have drunkenness, and unchastity at least implied(having it on scene is different). As well as officers living by what is to a large degree a pre-christian moral code? And of course it is impossible to write a mystery without murder.

Having specifically "Christian" stories often limits you to Sword and Sandals tales(which are wearing out their welcome), Amish love stories(which at least have a great staying capacity as no one expects plot variance in love stories), and Medievalistic romances and a few other genres.

This is exagerrated of course. But the point remains that a lot of the secularism of literature comes from the nature of literature, not from the philosophical or theological beliefs of the author.