Though even he ain't, the thing is we (western cultures in general) don't tend to particularly care until it stops being merely inadequate and starts being fairly egregious. Otherwise we'd be taking away kids below the poverty line left right and center and throwing their parents in the clink. Which we don't, mostly. If they're managing at least as well as a slum family barely not starving to death, either they should get a by or the slum folk shouldn't. Otherwise what you're trying to do has jack all to do with the kids and you should keep 'em out of the argument, heh.
Legally, there shouldn't be a problem re: multi-person marriage unless and until the folks involved start trying to wring tax and/or legal benefits out of it. Most of the practical reason many western courts don't provide a system for polyamourous relationships is because the legal and taxation issues involved are a complete fucking mess, and way more effort involved to work with than anyone particularly cares to invest. Divorce and guardianship hearings and whatnot are trouble enough with just two people involved, heh. Which is pretty much entirely an economic issue, with the interpersonal aspect (whether "just" a ceremony counts as a marriage or not -- it does, but not legally, which means sod all so far as religious or personal convictions or whatev' goes) being more or less entirely separate save for the issues that arise because of the economic entanglements.