Gonna ramble a bit about how I think Marriage works to Sangheili.
(Side note: Sangheili have many cultures across many planets, so there is no reason to presume that all Sangheili have the same concept for Marriage. For example, while it's commonly known that Sangheili don't know who their father is, the Ussans didn't have that concept, and commonly knew who their fathers were).
Sangheili have a concept of polyamory, specifically polygyny
Sangheili don't generally know who their fathers are (usually)
Sangheili practice wife-trading (as in, one Keep obtains women from another Keep).
Sangheili females generally have control over their marriage rights and mate selection, despite the wife-trading thing.
Sangheili do not have a concept of human concepts of divorce.
I want to make a headcanon ramble that ties all of these things neatly together.
First, there are two different terms of importance: Marriage, and Mate (in the sense of "Fira is Thel's mate"/"Fira is Thel's wife"). To many human cultures, these are the same. To Sangheili, these are two different things.
Marriage means that a Sangheili has entered another Sangheili's family, specifically for mating-right purposes. This is usually done via a female entering a male's family, but can also be a male entering a female's, or same-sex coupling. For the sake of convenience in this post, I'm going to refer to all scenarios as "She entered his family", which is the case like 80% of the time.
So, say Naz 'Shenen decides to marry into Shai 'Garan's family. As he is a low rank, this means that she has mating rights to him, his brothers, and his male friends. If she has an egg, she can declare that the egg's uncle is him, his brother, his male friend, or her brother. In the final case, she can either send the childling to go and be raised in her family, or bring her brother to Shai's family. If she does the latter, her brother can become a member of Shai's family as well. If her family raises the childling, then when he/she is of age, he/she can choose to rejoin the mother or remain in the raising family
If the person receiving a spouse is a Keep Elder, then the wife becomes a Keep Wife. A Keep Wife has mating access to any of the Keep Elders or the Kaidon. She is generally expected to not mate with any male inferior to the rank she married into. The exception is the Kaidon Wife, who is the Kaidon's personal companion and matriarch of the Keep Wives. Despite being married to the highest rank male who outranks the Elders, she can still mate with the Elders and is considered a Keep Wife in that regard.
The Kaidon generally is expected to get first dibs on any Keep Wife's first heat from when she enters the family, and any heat the Kaidon Wife has. He doesn't have to, but it's a courtesy.
The Sangheili word for "Wife" is different from the word for "Mate". "My Wife" should mean something more like "My family's wife". Human translators often translate them as the same word (ex. a human might hear a male Sangheili refer to his "Wife", when he actually used the word for "Mate".) Generally, the only time the Sangheili word for "Wife" is used to refer to an individual, is when it is spoken of or about the Kaidon: The Kaidon is the personification of the Keep itself, so all Keep Wives are his wives, and all Keep Wives refer to him personally as the Sangheili word for "Husband", although out of courtesy, Keep Wives usually state something like "Kaidon 'Vadam is my husband", while the Kaidon Wife would say "Kaidon Thel 'Vadam is my husband."
A "Mate" is a usually monogamous pairbond between two Sangheili. A Sangheili may declare another as his "mate" even if he has not fathered any children with her. Generally, "Mate" means less "only I can mate with her", and more "I love her / I am responsible for her emotional well-being." It is also common for a female Sangheili's mate to get first dibs on her heat, and therefore it is more likely that her offspring are fathered by her actual mate.
In very low-class, labouring families, like the one in which Fira was hatched, it's common to not dick around with the father's identity thing, since the females have better things to do than keep track of males or over-complicate things, so almost all childlings are just sired by their mother's mate, and they just know who their father is. If her mate dies or leaves her, or she has too many kids for him to mentor them all, she just assigns her/his brother to be an mentoring uncle. Generally, a male won't mentor more than 2 younglings/childlings at a time.
About 20% of Sangheili have two pairbonds. For example, Thel considers both Fira and Rtas as his mates, and Rtas considers both Thel and Thel's sister Lia as his mates (and obviously, this does not have to extend both ways: Fira is not Rtas' mate and Lia is not Thel's). They often point to spiritualism to support this: Pointing out "I have two hearts, and I give one to each of you." Or "As Urs has Joori and Fied/ Sangheilios has Qikost and Suban, so do I have you two."
(There are different Sangheili myths on how the celestial bodies are related to each other, such as:
All six of them are siblings
Urs is a Kaidon, Joori and Fied are his council of elders, and Sangheilios, Qikost, and Suban are their Keep Wives.
Like above, but Sangheilios is also a male elder.
Like above, but everyone is a male elder and there are no Keep Wives (less common. Most myths posit the moons as females).
Urs is in a gay poly relationship with Joori and Fied (less common myth, but commonly believed by gay Sangheili)
Urs or Sangheilios are in a straight poly relationship with Qikost and Suban
Urs and either Qikost or Suban are the parents of the other Celestial bodies.
Qikost and Suban are the mothers of all planets in the Urs-Fied-Joori system, and the three stars are the fathers/uncles/mentors.
Because Sangheili marriage isn't a 1:1 thing, divorce does exist to them, but looks nothing like what it looks to us. There are no laws locking one Sangheili to another, so if two Sangheili who are coupled decide to not be coupled anymore (what we would consider "divorce"), there is no big deal made. They just stop hanging around each other and he stops being one of her mating interests. She remains in the family she was married into.
However, if she decides to leave the family all together, it is a bit of a different story. A female who leaves the family she married into will generally:
Take her kids with her, regardless of how old they are. All of them lose right to that family, even if they are adults, elders, or the Kaidon himself.
Return to the family in which her mother currently resides (even if it is not the one that female was hatched in).
(If her mother is dead) Return to the last family that housed her mother.
(If she chooses not to go back to that family), enter a new family or start a new family.
If a male leaves a family he married into, he will:
Lose access to his kids and to any young he was uncling/mentoring. Adult kids might still want to stay in contact with him, but Younglings and Childlings are redirected to focus on their new mentor.
NOT return to his mother. He is expected to enter a different family all together, or start a new one. (His mother, however, may choose to join him).
And since marriage is not 1:1 to them, you can divorce your family without divorcing your partner. In this situation,
Your partner can choose to join you.
If a male leaves a family he married into, and his mate comes with him, and they have underage kids, he is allowed to return to his mother's family, and have his kids adopted into his mother's family.
Otherwise, the couple can also just start a new family or join a different one.
Wife-Trading is basically unheard of in any level of Sangheili marriage except Keep Wives.
A Wife-Trading situation is used to help strengthen alliances. Generally, the receiving family will offer alliance to the giving family, and the giving family may choose to demonstrate their welcome of the alliance by sending some of their Sangheili to join that allied Keep.
Not only wives are traded: males are also traded, although usually as military officers.
Regardless of if the trade is done of a male soldier or a female wife, the decision of who will be sent will be made first by a few mothers volunteering their sons or daughters, then the Kaidon and Council of Elders deciding, then the son or daughter confirming. The son or daughter is allowed to reject the offer, although they usually do not wish to disappoint their family in this way.
In Lin's case, she did not want to go to Vadam, but as her Keep, Refum, found it very important that the daughter of Sesa 'Refumee's sister be sent in their alliance agreement, to show that they are no longer mad at Thel for killing Sesa, Lin felt immense personal pressure to do it. But she could have said no with no real repercussion besides personal shame.
The purpose of the Wife-Trade is not just to give the receiving Keep more women, but to establish the giving Keep's blood in the receiving Keep, giving them more social power in that Keep. It is also used to demonstrate that the receiving Keep will respect the alliance, by making the woman the personification of the alliance. This means that if she is mistreated in her receiving Keep, her original Keep will take her back and consider this an act of war. They will also become suspicious if she is in the receiving Keep a long time without procreating: they will be suspicious that she feels unsafe or that the receiving Keep is not a good place to raise young.
Because mistreating a traded wife can cause a war, Sangheili in the receiving Keep do not take their responsibility to care for her lightly: if any male is seen abusing her, his own brothers' hands will be the first to tear out his throat, as they will see his abuse of her as an act of treason to their Keep.
Sangheili see being gay as little more than a personal preference, so have basically no thoughts on it. Since you don't need to know who your kids are anyway, there's not really anyone keeping track of the fact that a particular male happens to not have procreated with a female.
Because marriage is primarily just about belonging to a family, they also have no concept of gay marriage. If a male wants to marry a male of another family, then he just joins that family, his mate vouches for him, and he's treated just like any of the other males in the family, with no questions asked about why he shares a bedroom with another male. Likewise for females. Legally, they are seen as just having entered the family voluntarily, not specifically by marriage.
There is only one consideration Sangheili have about same-sex behaviour or coupling: If you're a male, you shouldn't be bottoming to someone ranked inferior to you. No one will stop you, they'll just think you're weird, in like a "chewing with your mouth open" kind of way.
