I read in a Heinlein book (so no "proper" reference to look it up) that someone had designated 6 sexes --
heterosexual male or female, homosexual male or female, bi- (or ambi-) sexiual male or female --
then in the book suggests three more -- omnisexual, solo (or narcissist), and neuter or asexual.
I think it was in Time Enough for Love, but could be something else.
So...does anyone know who suggested this and whether there is any published research/synopsis/whatever? Google is fail tonight, or maybe I just don't know how to phrase the query.