It appears you are making the mistake of presuming that a) this is representative of normative Greek beliefs about how humanity came to be and b) that even if it were so, that it has to line up with what we know in terms of science of the evolution of mankind in order to be “correct”.
It is no more factually “incorrect” than the Genesis story of Eve being created from the rib of Adam (but, interestingly, can be used to discuss the same core issues—why humans seek love relationships with each other).
Plato has Aristophanes telling this story in his “Symposium” in which a number of men of various sorts are sitting around discussing their theories on the origin of love as a change of pace for their usual drinking party.
classics.mit.edu/Plato/symposium.html. (here is an interesting discussion on the “Symposium” itself
condor.depaul.edu/~dsimpson/tlove/symposium.html)
It is worth noting that Aristophanes was a writer of comedic plays and a great satirist (puts me a bit in mind of Shaw or Jonathan Swift) and that I doubt Plato chose to use him as a character without reason or to have him be the one to tell this specific story without reason. The intent of this is philosophical, not a recounting of what Greeks believed about the literal way in which humanity came to be.
There are a great many different stories of the origin of man in Greek mythology.
encarta.msn.com/encyclopedia_761570116_3/Greek_Mythology.html has a sampling. Bear in mind that there is not and was not an established “orthodox” version of Greek religion, no infallible scripture dictated directly by a deity to exist unchanged throughout time. The stories are rather, as I see it, exactly that…stories, poetry, man’s attempts to describe in the limited human vocabulary their encounters with spiritual reality.