Additionally!
Both Polyandry and Polygyny were practiced in Tibet (Childs 72-74). Interestingly they also practiced Polygynandry (Childs 75), which i've never heard of till now. Polyandry was the NORM for relationships in Tibet, as any family with 2 or more sons would practice Polyandry, and with no sons but 2 or more daughters they did Polygyny. Only with 1 son did they practice Monogamy, and even then that wasn't set in stone as they could include a second husband in the marriage who was not blood related, although that was rare. I would chaff a little bit at the "I couldn't imagine a wife only having one husband" as monogamy was practiced in Tibet, and sometimes Polyandrous/Polygynous/Polygynandrous relationships would deteriorate into several sets of monogamous relationships (with the inclusion of another outside participant). Polygynous relationships were actually much less common than Polyandry (not gonna explain magpa in Tibetan culture, but you're free to look it up).
I'm not going to include much more data/writing on it as I've expressed what I want but I will include other citations that I would have used were I writing something longer.
Goldstein, Melvyn C. “Stratification, Polyandry, and Family Structure in Central Tibet.” Southwestern Journal of Anthropology, vol. 27, no. 1, 1971, pp. 64–74. JSTOR, www.jstor.org/stable/3629185.
Goldstein's paper discusses problems in ethnological research before 1971 as there were wildly differing reports on if Monogamy of Polygamy was more or less common, relating it to social and economical factors in Tibet.
Lau, Timm. “The Hindi Film’s Romance and Tibetan Notions of Harmony: Emotional Attachments and Personal Identity in the Tibetan Diaspora in India.” Journal of Ethnic & Migration Studies 36, no. 6 (July 2010): 967–87. doi:10.1080/13691831003643389.
BERREMAN, GERALD D. “Polyandry: Exotic Custom Vs. Analytic Concept.” Journal of Comparative Family Studies, vol. 11, no. 3, 1980, pp. 377–383. JSTOR, www.jstor.org/stable/41601144.
Childs, Geoff. Tibetan Transitions : Historical and Contemporary Perspectives on Fertility, Family Planning, and Demographic Change, BRILL, 2008. ProQuest Ebook Central.
Not in Tibet, but an interesting read and shows a more widespread inclusion of Polyandry in the region: Sommer, M. (2015-09-15). Polyandry and Wife-Selling In Qing Dynasty China: Survival Strategies and Judicial Interventions.
https://www.universitypressscholarship.com/view/10.1525/california/9780520287037.001.0001/upso-9780520287037.
Edit in to directly relate to what the guy said:
They made Kan Shiba think of the possibility that his fiance Moshi Rati have the ability to have one more husband than him which doesn't make any sense to me. It made me irritated beyond words. Worst chapter I've read yet.
I mean, she DOES have the ability to marry another man, and would be expected to if Kan had a brother :V
"It's not like my wife is mine alone after all" - Kan Shiba. She literally is. What a fucking laughable and stupid sentence from the author. Ruined my reading experience introducing fucking polyandry in the scene from a shitty cuckold side-character.
I'll just note that this ISN'T cuckolding, in like... every sense of the word?
And introducing polyandry is the correct decision for the manga. As much as the manga is about Kan Shiba and Moshi Rati, it is also about Tibet and its customs, and so it would be wrong for the author to deliberately leave it out. On the "my wife is mine alone after all" bit, it's more to do with the idea or sudden realization that he might end up with another man being married to Moshi. Kan Shiba is a young guy with no real experience in relationships, he's trying to figure stuff out and hasn't fully sorted everything out.
The side-character said "But there isn't any girl I'd expect that of"; of being monogamous? Are the author stupid?
I would say, as i've not opened up the chapter to recheck the scene, that she is saying I don't expect a girl to remain monogamous if she found someone else she also loved/etc.
Well, considering that Izumi is a girl names, then the author is surely a female and thought "what a fantastic idea to show that the women can decide whether they want one or more husbands in a formally, originally monogamous marriage set by the parents of the children by years back.
Izumi is not a girls name, not explicitly. It's unisex and both a first and last name. We also don't know the kanji construction of it from the English to tell what it is, as well as I believe it is a last name here?
Also, women or men choosing to change up their marital status was and is not uncommon in Tibet. It's a lot more dynamic than Western marriages.