"Both sides" as a definition, before you ever make comparisons, is already a diminutive of reality. The idea that there are only ever two viewpoints, despite the reality of the current American political situation, is naive at best. Even within the two major US parties there are fractious and conflicting personalities.
The "both sides" argument isn't asinine because it equates the two major players, its asinine because it accepts their premise that they're the only ones that matter.