zahavasdad: The people’s in the Andes considered it a grain and used it in a grain-like manner. Not so much as beets or spinach, but much like rice or maize (which we do consider to be kitniyos).
If you allow quinoa, it will have to come up with a justification to ban rice and maize (American corn). Thus the “bottom line” of the dispute is going to end up being not whether quinoa is kitniyos, but whether rice and maize are, and ultimately, should the prohibition of kitniyos be honored.
Based on biology and what Bubbie and Zaidie ate (okay, your ancestors prior to the 16th century), maize (American corn) should be permitted. So the dispute of quinoa is really more a fundamental debate on kitniyos in general.