Ok, lungman,

Here's why:

The anus is an orifice that leads to the anal canal, which is one to two inches (2.5 to 5 cm) in length. The anal canal leads to the rectum, which is about five inches in length. The first segment of the rectum generally angles towards a woman's navel, and then angles back towards the back slightly. It is this initial forward angle that causes the penis or dildo to stimulate the front wall of the vagina and G-Spot more directly than they do during vaginal intercourse.

And that's what "causes the O,"
The Love Goddess