This is often brought up in discussions of transgendered people, apparently to prompt the reaction, "Well, if those nice Polynesians did, than we should too!"
However, "recognizing a third sex" did not always turn out well for those so recognized:
"The Romans... regarded intersexed infants as supernaturally cursed and put them to death... [Among] the East African tribe, the Pokot... frequently, intersexed children are killed in the offhand way one discards an ill-made pot..." -- Clifford Geertz, "Common Sense as a Cultural System," Local Knowledge, pp. 81-83
Maybe the Polynesians were on to something here, and maybe not. My point is simply that citing what some other people in some other time and place did about some problem we are facing now is interesting, and can suggest avenues for further exploration, but by itself doesn't really decide much!