Sentosa, Akela Sathi (Lonely Companion). Themes are "elf brothel, wat do?" and a serious examination of the ramifications of the Akela Sathi system on eladrin society, relationships, marriages, family structures, child-raising, and expectations on both men and women.
Your move.
My own GM actually ran a significantly detailed side quest on this exact topic that spanned a large degree of homebrew content. I would like to see what others would do with the absurd subject handled in a serious fashion.
My GM and I did most of our brainstorming for Elfaivar when I had only the PHB avaliable (he gave me some additional infos), so he went with the Akela Sathi being a Sentosa-only institution and my character knew nothing about this tradition.
From the little that was known about Eladrin history, we deduced that Elfaivar must have been governed in a decentralized way with lots of local rulers and customs and a governing circle, the Ramandamandala, sitting in Bharata (it was called an Empire after all and I'm thinking in terms of the HRE or feudal Japan).
The development after the Malice fitted into this narrative with Lords warring and the whole empire falling apart, each region falling into chaos and trying to find a way out. One way, the PHB suggested, was Lords who tried to capture brides in order to become or stay powerful, and this destructive, oppressive way ultimately failed. I dubbed this narrative as "the mad Kings' era", a scary bedtime story to be told to little girls when they didn't behave.
The other, more successful path, was the founding of Enclaves centered around a surviving woman, who would be known as a Matriarch. I tried to find an explanation about how or why this would have been more successful, and came to the conclusion that it was 1) less destructive, as it was founded on growth and not on violence, and 2) that it basically made the Matriarch the "property" of the whole enclave instead of one power hungry guy. It was more of a bargain: she was given power and the position to lead, while she also provided hope for her people.
Combine this with far less people and the tradition of having a tradition of decentralized government one could easily understand why the enclaves would develop very divergently as every Matriarch had her own idea of what was best for her community.
But back to the Mad Kings. My GM took this hook to develop one lunatic King who managed to fly under the radar for quite a while, made some deal with a surviving devil of the Golden Legion and started an "empire" of his own. From what I know so far, it seems as if he used the pact to infuse several of his troops with fiendish blood and developed a strict hierarchy where he would give brides (captures, but maybe also some of his descendants) to his most loyal subjects. Hana Soliogn (Gale) is actually one of this King's descendants, so... that's where my character learned from his existence. Point is, he is aggressively trying to expand his territory, threatening Kellandia and the surrounding enclaves. So rallying them all and trying to mobilize a defense alliance had been one subplot that's been spanning half of the campaign by now.
Regarding family structure and sexuality in the enclaves, well, that's an interesting topic in my opinion (and I hope this text doesn't turn out to be essay-length when I am finished). As a first disclaimer, we (supposed westerners) tend to see fantasy societies and even other cultures through a modern-day, westernized eye. So we also tend to think that some conclusions are the only logical ones or that a society "of course" would develop in a certain way. I actively tried to shut that out when I thought about Eladrin society. I first tried to make a few assumptions:
- Eladrin society would have probably been egalitarian before the Malice. There is no historical hint that one gender was dominant, on the contrary: Kasvarina had been a wife, a soldier, and a successful single-parent mom of two girls who turned out to play important roles. The Ramandamandala also consisted of Lords and Ladies.
- Eladrin once were Elves who became fey due to their meddling with the Dreaming. It may be a bit of a fantasy stereotype, but I imagine fey to be less anxious when it comes to expressing their sexuality. And they'd probably be more likely to experiment as well. (I'm not even starting with the 5e Eladrin, who came much later, who can sometimes also switch their physical sex if they want to)
- Eladrin families most likely included one momogamous couple, even if we don't know whether they mated for life. (Kasvarina married Pillai)
Now the Malice struck, leaving only a few surviving women. And of course the question on how to survive and repopulize would arise in the Enclaves. Women who had been married to a man would most likely try to reunite with their husband in case he had survived the war. They'd try to have kids, sure, and the emphasis would be on having daughters to multiply, even if sons (who at least added up) would be seen as a blessing, too. In this scenario, I guess, very few couples would have allowed other men into their relationship. But we can only guess on how their daughter would find and choose their partners.
Lesbian or Bisexual women who had been partnered with a woman might have felt pressured to have kids and therefore sexual encounters with men. But there is no reason why they would threaten their society's peace by choosing only one partner. The Ananta Paudha philosophy of "serving" their community could have arisen from here with the Matriarch declaring all men to be eligible partners. The concept of fatherhood would be abandoned very soon in such an enclave, and, depending on the success of the concept, her daughters could continue "free love", leading to a community much alike "modern" matrilineal societies.
The same could be said about enclaves centered around a Matriarch who was single. Here, I assumed a lot of choices, but all lead to kind of abandoning fatherhood as a concept as seeing a child as someone calling a kid "my child" would easily lead to jealousy and strife in the enclave. Instead, children were offspring of the Matriarch, and all of the enclave looked after them. When her own daughters became mothers, they also became their primary progenitor. Here, I envisioned that some enclaves could develop several pathways: Ranging from young women running away with one man they love to women using polyandry but not fully developed partnerless reprodution to Matriarchs who saw all men (minus their brothers and sons) of their society as potential mates.
All of these societies would follow a matrilineal tradition, such as children derive kinship from their mother, staying with her and her society until they came of age to seek partners (and some, eventually, founding enclaves) of their own. And yes, jealousy or being shamed for one's sexual activity would probably be shunned upon. After all, women with an active sex life meant more children for the Enclave. Which could in turn mean that women might feel or be pressured to have partners, and have daughters. Which would make life reeeeally tough for asexual women.
Sexuality for male Eladrin would be a tough topic, but I'd think that the rate of bisexual men was much higher than in humans, so quite a few would simply resort to homosexual love. I also envisioned that violating a woman's sexual autonomy would be among the highest crimes against society as a whole (see my idea on the Matriarch as being "property of the whole enclave"), so violators would be punished harshly (most likely exile, but maybe even death).
I can also envision that young Eladrin men interested in women would, at first, try everything to get their attention, including said reckless displays of heroism. Which might be against the advice of their caregivers who saw them as valuable signs of hope (at least more valuable than the aging men of the surviving generation). Same could happen with eladrin veterans desperate for a female partner, even if many lacked the fire of youth.
Regarding Kasvarina's reaction: I saw her rage sparked by the same notion that made my character sick (well, she got a wholly different background and even accepting "randomly" born children made her confused). That the men had the "right" to have one sexual encounter once per year. Because that would mean they could ultimately demand sex. I didn't even think about Kas remembering prostitution as women serving men (it was an egalitarian society with no history of patriarchy), but as "people" (maybe even slaves) serving needy Eladrin.