Interesting. The first time I read about this was in Heinleins The Moon Is a Harsh Mistress. My personal opinion is that the construction should be legal but not neccessarily have the same custody and tax rules. The same rules is difficult anyway because you can't just ignore one part (the gender) but need to add lot cases resulting from extra element(s) present. The biological aspects (one biological father and mother) could be dealt with in analogy to adoption as is done with surrogates. But the many real life/law pragmatics of pair-relationships (marriage customs, custody, heritage,...) probably make it difficult to treat this the same way a 'traditional' marriage.
You would have to change a lot, but there is a natural basis to judge on: there's already the tax-law definition of 'household', and you could allow any or all of a household to file as a unit. It would also be very feasible to make a simple formula for the tax rate given the number of wage-earners in the household. The only trick would be dealing appropriately with teenager's first jobs.
Speculation is important for forecasting; it's also fun. Speculation is usually conveyed in two forms: in the form of an argument, or encapsulated in fiction; each has their advantages, but both tend to be time-consuming. Presenting speculation in the form of an argument involves researching relevant background and formulating logical arguments. Presenting speculation in the form of fiction requires world-building and storytelling skills, but it can quickly give the reader an impression of the "big picture" implications of the speculation; this can be more effective at establishing the "emotional plausibility" of the speculation.
I suggest a storytelling medium which can combine attributes of both arguments and fiction, but requires less work than either. That is the "wikipedia article from the future." Fiction written by inexperienced sci-fi writers tends to generate into a speculative encyclopedia anyways--why not just admit that you want to write an encyclopedia in the first place? Post your "Wikipedia articles from the future" below.