The pelvic floor has two inherently conflicting functions.

http://en.wikipedia.org/wiki/Pelvis

Pelvic Floor. 
The pelvic floor has two inherently conflicting functions: One is to close the pelvic and abdominal cavities and bear the load of the visceral organs, the other is to control the openings of the rectum and urogenital organs that pierce the pelvic floor and make it weaker. To achieve both these tasks, the pelvic floor is composed of several overlapping sheets of muscles and connective tissues.