They haven't settled mutant rights, or really dealt with the legal status of Inhumans or Atlanteans, let alone Deviants, Eternals, etc. so I don't think the UN in the Marvel Universe is all that up on this stuff.
Especially when dealing with the, like three or four 'machine people' out there. Quick Jocasta, Vision, Jim Hammond and Machine Man, use your vast lobbying arm to get Congress to legislate on this pressing issue that affect almost exactly 0.00000000000001% of their constituency!
I blame the Latverian ambassador for constantly spoiling the votes.
Seriously though, as of the eighties, when the US govt kidnapped and mind-wiped the Vision, it seems like 'machines don't have rights' is the rule. If Wanda and Vision are 'legally' married, it might have been a one-off thing, officiated by an especially open-minded clergyman or city official, and not something that the federal government ever really saw fit to either recognize or dispute (and since it ended without any sort of legal divorce, it seems that the official response of 'ignore it until it goes away' was the economically sensible choice).
There was no political benefit to either recognizing, or standing opposed to, this, until now, so I suppose it makes sense, in a world full of gods, aliens, mutants, etc. to let those half-dozen robot-people just do their thing. Now that there is an actual movement, there will be more of a call to do something about it, and, no doubt, some people asking this very question, 'why wasn't anything done until NOW?'
Plus, someone somewhere probably thought, "Do we want to kick this hornet's nest, and possibly give *Ultron* 'human rights?' Or do we prefer a world where his legal status is 'blow his genocidal toaster ass up.'"