those are the rules for federal tax law in america, on earth, where only human beings can be married to a single other human being at any given time
if we are assuming that in space it is possible to be polyamorously married to various forms of alien, humanoid and otherwise, with variable lifespans and definitions of intimacy, we must also assume that the rules around the tax laws created to incentivize marriage must also be different
‘married filing jointly’ makes very little sense for situations where you are married to three people who each have their own spouses who may not be married to you, and furthermore the tax status is only necessary if what you are trying to encourage is long-term monogamous relationships (which i don’t think the empire or the republic particularly care about). and dependency rules about co-habitation do not make sense for species whose biology or culture negates the possibility of co-habitation even in closely intimate relationships (and definitely doesn’t make sense if someone’s job requires them to spend most of their time traveling through space hauling cargo, or if the government has mandated they work on another planet for some unspecified period of time)
(there is also no meaningful definition of ‘annually’ in the context of space taxes, and therefore taxation periods must be defined per-planet as lived on by the head of household)
(we must also assume that each planet has its own tax structure, and therefore what we are worried about here are republican or empirical taxes, or as they are colloquially known, ‘space taxes’, the taxes you pay to the space government as opposed to your planetary government)
in theory we could assume that the space government simply doesn’t incentivize marriage, because why would they, but that doesn’t work for fic purposes. therefore the most logical reason for the incentive is liability. in that case, each marriage would define one person as the head of household, and the other as a dependent–with the head of household being the person who is legally liable for the other’s taxes and whatnots. if a HoH also has legal access to the assets of their dependents, in order to maintain the household, this creates a set of checks and balances (as it were).
the person in a marriage defined as head of household must therefore be someone that the dependent trusts to be able to keep their shit in order, and the person defined as a dependent must be someone the head of household trusts not to totally bail on them with a bunch of federal property. who’s who therefore becomes a personal choice between the married individuals.
if we assume this stacks, then let’s say person A is married to person B, and person B is also married to person C. if person A is HoH in the first marriage, and person B is HoH in the second marriage, person A still gets access to all the assets of person C as the dependent of their dependent. this means if your husband is thinking about marrying some fucking rando, you’re incentivized to make sure everything is on the level so you don’t have some shady motherfucker with complete access to your assets, or alternately, the ability to make you legally liable for serious space crimes. this is the primary disincentive for fraud–marrying someone who wants to commit fraud is a fast track to either getting all your shit stolen or else ending up in jail for a crime you didn’t commit.
alternately, if lando is married to han, and leia is married to han, and lando and leia are both HoH, things get theoretically complicated. things can get split up according to various formulas, or one of them (leia, it’s leia) can claim more limited benefits in exchange for giving up the majority, as well as surrendering access to han’s assets or liability for his dumb horseshit (”don’t look at me, call his husband, i’m not responsible for that dingus i just have the option to be. you think i want access to his checking account? he’s got three dollars and a pack of gum in there.”). marriage in that case is more a matter of having familial access to your spouse (hospital visitation, etc).
the majority of incentives (in the form of exemptions, credits, etc) would be for the HoHs of dependents who do work the government particularly needs done, because government contractors are the ones the government is most worried about bailing off to nowhere planet with a bunch of stuff. a liability-based system makes it possible for the space government to go to their spouse like “hey… your husband took off with all our shit, pay up please”. therefore having certain kinds of dependent would alter the type of HoH someone is in order to determine what benefits they receive and what liabilities they are assumed to have taken on.
that’s the logic i used, anyway