I read this series when I was younger but cannot remember the name to save my life. I remember a species of Aliens saves humanity from the ashes of an apocalyptic nuclear fallout and in the resulting event humanity is rendered essentially sterile.
The species has three genders: male, female and ouloi. The ouloi are required for humanity to procreate.
That is all I remember of it.
I believe you are looking for the Lilith's Brood books, by Octavia Butler.
(Formerly printed in a collection titled 'Xenogenesis', but no longer in print under that title.)
From the Wiki article:
Lilith's Brood introduces the alien species, known as Oankali, who come in three sexes - male, female and ooloi, a sex that mixes and manipulates the genetic material produced by the other two.
Interesting, if very bizarre books. Not for everyone.. but effective at exploring some unusual concepts in psychology and sociology.
