Four-fingered and six-fingered hands are common among their species. Five is a genetic trait associated with powerful psionic talent, and the taboos are associated with that... and the book *does* force the characters to dig into the distinctions between themselves and, say, a member of a different five-fingered species.

Like I said, there's depth to it. This is also the first book I saw that presented the idea of True Soulmates and then (explicitly) said, look, this isn't *love*. Soulmates can wind up lovers, mortal enemies, or just destroy themselves trying to connect. The bond doesn't care.