In pre-Christian Scandinavia it was considered Ergi (Womanly) for a freeman to have sex with another freeman; Wealthy norse men would keep concubines, which were women he would have sex with who weren't his wife, they were almost always of the slave cast.
Slaves are not freemen (obviously) so if a Scandinavian nobleman were to take a male slave as his Concubine, how would that be viewed by their society?
Tl;dr: It could probably be tolerated for freemen to have (one-time?) same-sex relationship with their own male thrall, just as they did with female thrall, but this would not meet the primary (though not all) social function of "concubinage" (frilla) in [pre-Christian] Old Norse World.
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First of all, check /u/sunagainstgold's seminal answer on the Old Norse homosexuality in this subreddit: How did Pagan Vikings in the 8-13th centuries view homosexuality?. Mainly based on Preben Sørensen's book Unmanly Man (1983) and 12th century Icelandic penitential, she highlights the point that the active partner of the same-sex relationship could also be shunned, since he was likely to violate the other person's manliness/ honor by committing such an act. She also emphasizes the margin of uncertainty since virtually all the relevant primary sources only dates from Christianized period.
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Then, how about the sexual relationship/act with thrall?
The key text, not mentioned in the linked thread, is the Chap. 198 of Older Gulathing law (from Western Norway, before the middle of the 13th century):
"Everyman has the same right to compensation for sexual intercourse on the part of his slaves as on the part of his bondwoman.
Compensation shall be paid according to the right of the king's representative if someone mistreats [that is to say, sexually abuses] a king's slave working in the king's courtyard.
A slave enjoys the protection of a ring (baugr) when he accompanies his master to the assembly or to church or to an alehouse when he is ordered to go; for in those three places every man's right to compensation (in case of insult) is doubled.....(Simensen trans. 2021: 158)."
Gade interprets this passages as following, citing another law clause (V-20) of Frostathing Law (Late 12th century Central to Northern Norway) that permit the owner even to kill his own thrall without any punishment as long as he makes the act of killing in public the same day:
"Sexual intercourse between two freemen, in which one played the passive role, was considered derogatory, whereas there seems to have been nothing derogatory in a slave playing that part. Only when the property of a master was violated (just as in the cases of the man's wife as well as female kindled), did it became a punishable act and a part of the Old Norse penal code (Gade 1986: 136)."
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Then, on what part(s) I might object about OP's concept of taking "man concubinage"?
Older Gulathing Law also has another unique passages on the same sex sexual act/ relationship between two (free)men (Chap. 32) in Old Norse law texts:
"If two men have have sexual intercourse and are identified and convicted of this, then they are both úbótamenn. And if they deny (it) while it is still rumoured in the district, let them confirm their denial by carrying hot iron. But if they are found guilty, then the king is owed half of their property, and the bishop the other half (Simensen trans. 2021: 40)."
[added]: úbótamenn (óbótamenn) means severe criminals (usually outlawed with their possessions confiscated), without any possibility of paying compensation by money in the legal settlements.
While scholars disagree about the details (exactly when the passages were added to the law book), at least they agree that these passages had not been a part of the law book before, and was a later addition (its terminus post quem was 1160s). Thus, it is difficult to ascertain the spread of such sexual practice in the society almost solely on this clause (Note that the corresponding passages also would disappear in later medieval Norwegian legislation).
Wealthy norse men would keep concubines, which were women he would have sex with who weren't his wife, they were almost always of the slave cast.
While some scholars certainly suppose the usual form of male-female concubinage as well as the social origin of concubines so as OP does (Brink 2012: 178, citing Tore Iversen's seminal monograph on Old Norse slavery), a few recent Icelandic studies instead shed light on hitherto often neglected aspect of the concubinage primarily as a social bond (a kind of patronage) between the influential man on one side and the concubine's family of lower social status (Jón Viðar Sigurðsson 2017: 21f.; Cf. Beck 2007: 111 - Auður Magnúsdóttir's dissertation, Frillor och fruar: Politik och samlevnad på Island 1120-1400 (2001) is the pioneering work). To give an example, Jón Viðar Sigurðsson summarizes the alleged characteristics of such a relationship between the chieftain and his concubines in Christianized Iceland as following:
"In the period from the end of the 1100s until 1262-64, the vast majority of chieftains had relationships with several concubines at the same times. These relationships were entered into with the consent of the girl's families. Concubinage was advantageous for all the parties involved. The girls secured their position in society. It was better to be the mistress of the chieftain than to be married to a poor householder. Also, this relationship gave the concubine's families extra protection, and if sons were born who later became chieftains, this would add luster, and possibly riches, to the concubine's families. Through this relationship, the chieftains secured loyal support for themselves from important local actors (Jón Viðar Sigurðsson 2017: 22)."
In such a way, male-female concubinage had some apparent "benefits" for the concubine as well her family so some socially lower women agreed to have the relationship with the influential man. As for the male-male relationship (concubinage), to what extent these "benefits" also applicable to the passive sexual partner (male concubine) of freeman in Viking Age/ Medieval Scandinavia?
Thus, unless different merits of social protection from the active partner apparently surpassed the loss of local social reputation (ergi), there was little to none incentive for any freemen to accept the proposal of being male concubine to another freeman at least of their free will, I suppose. Concubinage in Viking Age/ medieval Scandinavia was not just an one-time fulfillment of carnal desire, but also had an aspect of building the social network between two partners involved in longer term.
References:
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