Many mammal species practice forms of sexual coertion in reproduction.
Several species (ie. cats and weasles) depend on pain (neck bites, barbed
phalluses, etc.) to triger ovulation in the female.

In most social Primates species only dominant males are allowed to mate.  To
maintain this dominant posistion, alpha males in some species regularly
mount subordinant and immature males and females.  No human BDSM Master ever
ran a tighter harem.

One notable exception to this trend is our closest relative. the Bonobo.  In
this species (formerly known as Pygmy Chimpanzies), females only pair-bond
with other females.  Thus in any head to head confrontation, it's always two
to one, the females dominate.  To make up for his lack of status, the male
Bonobo is rewarded by constant sex (avg every 20 minutes) with females that
will mate even when not in esturus (humans are the only other female
Primates that will do this).  Menage et trois is de reguire (pardon my
misspelled French).

Ray Audette
Author "NeanderThin"
http://www.neanderthin.com