The "love that dare not speak its name," which Oscar Wilde shared with Lord Alfred Douglas was cited at the former's trial for gross indecency.  Accepting homosexuality as morally permissible has often been cited by conservative moralists as providing a slippery slope to Gomorrah, setting precedent for even lewder acts, such as bestiality or incest.  However, regarding incest (but I'm betting bestiality, as well), its lure seems to have been with us as long as homosexuality.  If not always accepted in practice, incest is a longstanding part of mankind's fantasies as a seedy imaginative otherworld, suggesting what's always possible if man-made laws didn't get in the way.

Greek deities and demigods, for example, were a saucy bunch: Zeus, the longest running head of the Gods, was the son of brother and sister Titans, Chronos and Rhea.  Following in the family tradition, Zeus's second wife was also his aunt Themis, goddess of law.  After things went south with that, he hewed even closer to his father's matrimonial views and married his sister Hera, who gave birth to Hephaestus, buttfugly God of blacksmithing.  Hera, being the Goddess of chain-smoking trailer trash with a thing for two-timin' goodfernothins, had little need or love for such a ghastly son and kicked the poor fuck out of Olympus.  Despite this treatment, according to some versions of the myth, Hepahestus sided with his Ma's henpecking his Pa, resulting in Zeus beating the tar out of him, giving him an eternally permanent limp.  Those kind of mommy issues point towards meth addiction and a life of petty larceny, if these had been mere mortals.  But they weren't, so Hephaestus managed to marry the most beautiful of all the Olympians, Aphrodite, Goddess of love, who was also his half-sister by way of Zeus's tryst with Dione.