After seeing the Assassin’s Creed gameplay I’ve been curious.
I have an earlier answer to this question, if you're interested!
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The first thing to keep in mind: basically all our written evidence comes from the thirteenth century, after Christianization. The Icelandic Homily Book and the penitentials (lists of sins and prescribed penances) that come down very harshly on homosexual behavior altogether, in exactly the same way it is being condemned in in continental Europe at the same time, are actually some of the earliest written Norse sources.
Then there is some interesting linguistic evidence from the Gulathing Law, which is a bit older probably still post-dates Christianization in its first transcription.
[Relevant to the earlier question in particular]: When talking about same-sex sex in the Middle Ages, it's important to consider a factor besides the assigned-at-birth sex of the people involved. According to scholar Preben Sorensen:
One of the questions that will be raised in what follows is the extent to which it was only the passive role that was contemptible; or whether homosexuality as such was condemned, in accordance with Christian moral teaching. It can be said at once that the answer is both/and...The active part must also have been condemned, not only from a Christian point of view. (27-28)
I don't think it's possible to give a definitive answer about pre-Christian Norse society based on the later evidence. However, comparing standard medieval Christian teaching (as expressed in the Homily Book and "Bishop Thorlack's" penitential from the late 12C) to the secular law and saga evidence can give us an idea of what we might call the extra-Christian, if not pre-Christian, culture of high medieval Iceland.
In the penitential, the very first of the "cardinal sins" listed is the sin against nature. It prescribes 9 or 10 years of penance for men who commit illicit sex (the word is adultery) with other men, or with four-footed animals. As John Boswell points out, this penance is exactly the same prescribed for men who rape women. One of the Homily Book's sermons agrees, cagily referring to "those appalling secret sins perpetrated by men who respect men no more than women, or four-footed animals."
This is solid high/late medieval Christian teaching. Any non-procreative sex act is "against nature," with sodomy and bestiality accounted the worst. All parties involved are to blame (it was common practice into the early modern era to also slaughter the animal--including, by the way, in colonial America). The emphasis, basically, is on sex.
Whereas Christian theologians tried desperately to talk around sodomy, Old Norse gives us several words that circle around a semantic field of male-male sexual behavior. I want to talk especially about argr or ragr. Its primary meaning is "cowardly, unmanly, effeminate, with regard to morals or character" (Sorensen 20). A secondary meaning is, essentially, a man who is inclined to take the passive role in sex with other man.
So you can see a couple of things. First of all, that being the receiving partner in a male-male sexual encounter is considered the height of unmanliness. It is the same word as "effeminate;" it's equated with cowardice. There's no question that there was a massive stigma attached to that status; that this was a phrase slung at people rather than claimed proudly or even used neutrally.
But what kind of stigma? The application of the feminine form, org, to women can start to clue us in. Rather than referring to lesbianism, or to having PIV sex with a man, it's roughly equivalent to modern "slutty"--including the derogatory nature. We're dealing with violations of muscular masculinity and chaste femininity, of course, but at the same time--and inseparable from the gender roles--these acts are violations of fundamental personal honor.
The transgression of masculinity and of honor comes across quite strongly in the legal texts. The Gulathing Law, which has its roots in Norway before Iceland, links three offenses:
One is if a man says to another that he has given birth to a child. A second is if a man says of another that he was demonstrably [f--ed] by another man. The third is if he compares him to a mare, or calls him a bitch or a harlot, or compares him with the female of any kind of animal.
Couched between the two very clear insults of effeminacy by proxy, the implication of being the receiving partner in homosexual male sex is that it turns the man into a woman from the viewpoint of social standards. It is a violation of gender roles. This is rather different than the Christian theological view that hinges on procreation.
High medieval Icelanders, therefore, placed a premium on the sharply delineated gender roles and morals of their society. Mark Jordan points out that on the European continent, the rise of concern over sodomy from the eleventh century comes out of the need to impose social order, especially through marriage. So when Sorensen talks about how strictly gendered Icelandic society was and Auður Magnusdottir emphasizes the importance of marriage as a political act, it seems that disgust with the passive partner in male homosexual behavior involves his perceived threat to social order.
So all this seems to point to a lack of concern for the "active" man in the partnership. Well, as quoted above, Sorensen says not so fast, and I'm inclined to agree.
You'll notice that this passage doesn't actually involve a sex act at all! (In fact, it's rare to see persecution for actual sodomy acts before the late Middle Ages on the continent, even). Rather, it describes the accusation against someone else. The word I translated above as f--ed is sorðinn (or sannsorðinn), but there is another less coarse and more complex ring of words based on nið. nið can mean degrading slander, although, with the close relationship of gender and honor in Icelandic society, there is usually a connotation of sexual depravity involved in the accusation. In the sagas, nið can be a threat as well as an action; it can be physical (erecting a monument to a man's unmasculine behavior, especially cowardice) as well as verbal.
And, in the thirteenth century law codes, nið can be a crime. If it explicitly involves the words argr or sorðinn, the slandered man has the right to kill his slanderer in retaliation. It is, the laws clarify, exactly the way a man has the right to kill on behalf of a female relative whose honor has been violated.
The point here is that violating someone's honor is a despicable offense--meriting a fine, meriting outlawry, meriting a duel, meriting the death penalty. True, the laws concern verbal or monumental accusations. But as we saw earlier, to be the more passive partner in a male homosexual encounter was to have one's honor as a male violated. Thus, to be the more active partner is to violate the other person's honor. This, the laws concerning slander make clear, was utterly unacceptable to medieval Icelanders.
So it's not a surprise to see, in the sagas, not so judgmental references to castrating one's enemies, or to raping both women and men subjugated in war. The conquered--whether it's an opposing army or your son-in-law's family--already have no honor to violate. But two men in the same village? That was a different story.
Further Reading:
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I've edited the answer a little since it was originally aimed at the expanded text of the earlier question--please let me know if anything is unclear!
https://www.reddit.com/r/AskHistorians/comments/5vb166/comment/de1bo9q
Check out this answer by /u/sunagainstgold that answers a similar question!