Whatever set of tags one proposes for generalized marriage, it's helpful to run it trough a series of scenarios to see how those could be addressed. And maybe a scenario or two that can't be addressed with a justification why (it's icky, it'd require seven-layer-deep token trees, Toady has carved a slab stating This Shall Not Be, etc.)
Some ideas to keep in mind:
Should marriage tokens refer to castes, sexes, social ranks, marital statuses, etc.? For example, nobles can have concubines, but concubines must be untitled at the time they "marry" the noble. Acquiring a title would dissolve the concubine's "marriage" instantly. For another example, perhaps the Champion is permitted to adopt anyone he/she chooses into the Champion household to enjoy the lavish furnishings and such, but this is completely independent of the Champion's marriage.
Courtship is currently only possible if marriage is plausible (although one or both partners might have commitment issues that prevent actual marriage). Should it be up to the entity file to describe every possible form of deviant courtship (affairs, romancing a forbidden caste, etc.)? Or should every biologically possible type of courtship automatically appear every time? Courtship is currently also fully consensual, but that doesn't cover every possible arrangement. Is it too icky to go into unwilling matrimony?
What is a rough sketch of the data needed to track lineage? Will the game need to generate group IDs for families? Is it important if another spouse is added to the family after the child is born? Anything beyond strict monogamy and Bonobo Nation will require a notion of what the family looked like at a specific point in time.
Should marriage tags influence things like target family size? Might it be important to have nominally identical institutions that vary only in target family size or some other parameter? For example, suppose nobles and commoners and serfs all have "serial monogamous marriage" but they vary in target family size and the seriousness of infidelity. Is that kind of variety worth the coding and simulation complexity?
How does an entity treat a member (presumably an immigrant) who is incapable of that civilization's legal marriage forms? Do they get a pass (or at least reduced punishment) for an arrangement they can use? If we go with pre-defined forms of deviant relationships, such an alien might find itself driven to "unthinkable" acts in that social context. A simple version would be that these square pegs are doomed to perpetual bachelorhood, but this really underplays the importance of sex drive in personal motivations. And what about a civilization with an ineligible member yet mandatory marital roles?
Do preferences for certain relationship types vary by age, professional status, social rank, etc? Which stats, personality traits, values and ethics affect this? The current system has a simplistic scale of marriage, lover only and hell no. So this is beyond what's legal in that society, more to what people would do if they had the choice. By extension, at what point does a marriage-minded person give the commitment-challenged partner an ultimatum? Can the system allow an extramarital affair to evolve into divorce and marriage? In more exotic combinations, at some point a concubine might insist on full spouse status or walk.
This kind of thinking will hang some "why" around proposed tags that can be debated with at least some shared understanding of the author's intent.