An alien species' sex system is very bizarre in comparison to terrestrial ones. Perhaps it has two sexes that are not identifiable as either "male" or "female" in terrestrial terms, or perhaps it has three or more sexes, each of whom is indispensible for the species' reproduction process.
Of course, Earth has a few bizarre examples of its own. Some species of fungi have more than two sexes [...] [tvtropes.org]