Sexuality in Medieval Europe: Doing Unto Others by Ruth Mazo Karras is a good overview of the subject (love that title, btw), and Michael Rocke's Forbidden Friendships is an excellent case study of Renaissance Florence. The rest of this post is copy-pasted from my dissertation (in progress). It specifically describes 15c Florence, but it generally applies to the entire Christian era in Europe, at least up until the 19th century, when the term homosexual was first coined:

In our case, we cannot accurately call men who had sex with men in Renaissance Florence “gay” in the modern sense, since this aspect of human sexuality was not considered an essential or innate element of one’s identity—not to mention, perhaps more importantly, that men who had sex with men generally did not do so exclusively, and would thus fall short of modern standards of gayness. But we must also remember that just because something is socially constructed does not mean it is not real: it has what Machiavelli would call “effective truth.” We certainly cannot say that men who had sex with men in the Middle Ages “didn’t know” they were gay, nor does it imply that men who identify as such today are “not really” gay. Instead of the modern dichotomy of heterosexual and homosexual, there are three axes that are much more useful in conceptualizing Medieval sexuality.1 The first is between sex and chastity; the second between reproductive and non-reproductive sex; the third is between the active (penetrative) and passive roles in sex.

From the most conservative, Christian point of view, sex is inherently sinful, and only remotely excusable when performed by husband and wife for the purposes of having a child. While many conceded that it was also acceptable to have sex in order to please your spouse and/or prevent them from seeking satisfaction elsewhere, some preachers maintained that even this should be avoided. While a complete discussion of the complex theology and philosophy of chastity in the Medieval world is too much for this space, suffice it to say that there was a powerful rhetoric on the absolute sinfulness of sex that pervaded society, resulting in complexes of guilt and shame no doubt still familiar to many modern Americans who grow up in conservative households. Ruth Mazo Karras argues, however, that chastity was a much more powerful compulsion in that period, as a significant portion of society pursued it as a way of life. Karras goes so far as to call chastity in Medieval Europe a kind of sexual orientation in the modern sense, because it was what she calls “an erotic chastity. That is, it did not represent a lack of desire, or a lack of opportunity to satisfy desire, but a more or less deliberate orientation of desire toward matters of the spirit.”2 There was very little room in the public imagination for those who pursued sex outside of marriage: medieval terminology divided women into the virgin, the married, the widowed, and the prostitute. A girl who had never been married was assumed virgin, and if found to be otherwise was in danger of being labeled a prostitute; there was no term for what we would call a “single woman,” as a sexually active and independent person.3 Widows were a different animal, and much more threatening to the social fabric. It was generally assumed that a girl’s innocence of sexual matters would protect her virtue, but once exposed to sex in the marriage bed, women became insatiable. A woman who knew what sex was like but had no man to occupy her was a hazard to the men she might seduce and the girls she might corrupt. Men seem to have had much more license in their youth to vent their desires with prostitutes and girls of lower social standing, but they did not escape the admonitions of their preachers, and many men did commit themselves to lives of chastity and self-flagellation.

Those that were not so pious as to insist that all men and women refrain completely from sexual activity still generally thought that it should remain within certain bounds proscribed by God and nature. Any activity that deliberately obstructed the potential to conceive might be censured by the preachers of the day. Masturbation and the time-honored practice of *coitus interruptus* received the label of *onanism*. Any insertion of the penis into an orifice that was not the vagina was *sodomy*. Indeed, while male-male sodomy was certainly more shocking, provoked greater public response, and received harsher official censure, male-female sodomy did not escape the attention of church and government officials. Michael Rocke has identified a number of cases from fifteenth-century Florence in which married couples were indicted for practicing anal and oral sex in order to prevent conception. Men could also be charged for sodomizing a female prostitute, and in cases of rape or abuse, the addition of a charge of sodomy made the punishment considerably more severe. Niccolò Machiavelli himself was brought to the attention of the *Otto di Guardia* for relations contrary to nature with a woman (probably a prostitute) named Lucrezia between 1514 and 1516. What this means is that a man who had casual sex with men would not consider himself fundamentally different than a man who had casual sex only with women, though he may have recognized that he was committing a “worse” form of sodomy. More importantly, although Bernardino of Siena and others claim that there were men in Florence who were so addicted to sodomy that they were wholly repulsed by women, the average sodomite would not have felt that his desire for boys precluded him from also desiring girls.4 That identity crisis common to many straight, American men—who may find themselves perplexed and terrified by the stirrings of attraction to fellow men, by their physiological response to the sculpted and virile bodies of the male performers in straight pornography, or even by their strong emotional attachments to close friends—would have been largely alien to the Florentine man.