“Boy” is only the opposite of “girl” if you’re working within the framework of gender binary. One could equally argue that “non-binary” is the opposite of both “boy” and “girl.”
Similarly, “Right Libertarian/AnCap” and “Tankie” are opposite ways to justify authoritarianism and atrocities (dictatorship of the “market” vs dictatorship of the proletariat). “Anarchist/Left Libertarian” is the opposite of both because both assert that non-consentual hierarchies are normal and excuse the atrocities that come with them while anarchism rejects the validity of non-consentual hierarchy.