Parthenogenesis is a form of asexual reproduction (sort of a 'last card to play', biologically speaking), in which a female is able to reproduce without fertilization.
Thing about this 'theory' is that Parthenogenesis usually produces females, at least in XY-based organisms.
Since ZW-based ones can actually produce males, though, there's no reason to think there's an Alien chromosome thing going out there, so one can actually buy that theory. So it's between this and Aliens being hermaphroditic.