One important primary source for medieval opinion of note directly addressing homosexuality is Alain de Lille's Plaint of Nature. This being a late 12th century work.

Alain addresses a number of points which conventionally come together to constitute the pre-modern Christian objection to homosexuality. That homosexuality implies a rejection of the male sexual and gender role. By extension, that homosexuality represents a descent from higher to lower station. Moreover, that the sexual act, in a homosexual context, constitutes onanism, or waste of the seed.

To wit,

The sex of active nature trembles shamefully at the way in which it declines into passive nature. Man is made woman, he blackens the honor of his sex, the craft of magic Venus makes him of double gender. He is both predicate and subject, he becomes likewise of two declensions, he pushes the laws of grammar too far. He, though made by Nature's skill, barbarously denies that he is a man.

Now, obviously, he's also making the argument that homosexuality is unnatural in the broadest possible sense, contending that it is against the nature of even grammar and verse, by which we collect, convey and employ reason and knowledge. As odd as it may seem to us today, that Latin grammar should be the basis for constructing a model of appropriate sexual conduct.

hi! there have been several posts on homosexuality during the middle ages; you may find these of interest

• How did homosexual people in the Middle ages, in Europe (be it peasants or nobles) find other homosexual people if it was so frowned upon? I'm sure they didn't have Grindr, so what did they do?

• Are there any noteworthy example of tolerance towards homosexuality in the Medieval Age?

• During Medieval Europe, how taboo was incest and homosexuality comparatively, and to what extent if at all?

• Are there any medieval rulers that we know were homosexual?

• Were monasteries/ nunneries for gay people?

• Is there any evidence of homosexual activity amongst medieval European monks?

• Medieval monks and homosexuality

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