From Metamorphoses of Science Fiction, pp. 24-25:

A further step down into pseudo-sophistication — correlative, no doubt, to a marked decadence of cultural taste in bourgeois society and its literary markets — is the parasitism of Gothic, horror, and weird fantasy upon SF. … One can understand some readers’ panic flight from a science which produces nuclear bombs, napalm, and nerve gases, from a reason which justifies class societies in mutual balances of terror, condemning two-thirds of the world to hunger and disease, and the remaining third — “hypocrite lecteur, mon semblable, mon frère” — to the boredom of a nine-to-five drudgery relieved by flashes of TV commercials. Maybe such readers ought to have an escapist enclave of sword-and-sworcery or Cthulhu cosmologies — I cannot say. But surely SF, built upon the premise that nature is neither a childishly wicked stepmother (“As flies to wanton boys are we to gods / They kill us for their sport”) nor inscrutably alien to man — surely SF cannot allow its contract with the reader to be contaminated by the Great Pumpkin antics of fantasy. Even more perniciously than is the case with the bland fairy tale structure, the black ectoplasms of fantasy stifle SF completely. Its time shrinks to the point-consciousness of horror, gloom, and doom, its daydreams turn into an inchoate nightmare, and under the guise of cognition the ancient obscurantist enemy infiltrates its citadel. Fossilized fragments of reasoning are used to inculcate irrationality, and the social energy of readers is expended on Witches’ Sabbaths instead of focusing it on the causes of our alienating, murderous, and stultifying existences: the power structures holding back the hominization of the sapiens, the true demonology of war and market breeding pride and prejudice.