Christianity has long been homosexuality's great enemy
See more » Love and Lust
From a really fine roundup of recently published gay history books.
The classical preference for pederasty vanished with the conversion of Constantine to Christianity. Suddenly the polite debates about boy love versus heterosexuality were replaced by legal persecutions of homosexuals. Constantine's own sons passed stringent laws against sodomy, and these were codified and expanded under the Emperor Justinian. As Edward Gibbon wrote in the 18th century, "Justinian declared himself the implacable enemy of unmanly lust and the cruelty of his persecution can scarcely be excused by the purity of his motives.... " Men — even bishops — accused of sodomy had their penises removed and were paraded naked in the streets.
In the Middle Ages the punishments were still harsher and more widely applied. Many sodomites were burned to death. In Ireland consent wasn't even an issue. One Irish penitential decreed: "A small boy misused by an older one, if he is ten years of age, shall fast for a week; if he consents for twenty day." Under Charlemagne the laws against homosexuality were conflated with those against bestiality (quite literally homosexuality had been "de-humanized"), and a church council at Paris explicitly endorsed capital punishment for homosexuals. The council interpreted the writings of St. Paul as advocating the death penalty — in the Epistle to the Romans, Paul had found "this infamous crime" to be "worthy of death."
Edmund White, The Los Angeles Times: The history of a love that dares speak its name