I would say that Morrison used such a strong array of sexual imagery in the novel for several reasons. First, passion in general plays a strong role in all of Morrison's work, and sexual desire is a powerful passion. Second, there is a longstanding stereotype about Black sexuality, and I think Morrison is playing against and exploding that stereotype. Another way she's writing as a Black writer is by writing about how the African-American position in society places undue weight on people, distorting them; the odder sexuality in her work is sometimes evidence of this. Finally, Morrison is writing something like modern myths or American magical realism, and uses the sex as a kind of barely naturalized magic.