Someone who is only sexually interested in the
opposite sex and does not recognize gender as being separate from
biological sex. They still may use peoples requested pronouns, but this would be as a curtesy not as a belief for them.
Originated as a play on words with the term "Sith Lord" from Star Wars, utilizing the related "rule of two" to refer to the belief that there are only two sexes(genders) as apposed to the master apprentice "rule of two" from the movies.