In human embryo development, when gastrulation occurs, just after cleavage, the outer cells roll into the interior and create a tube with an opening at both ends. The first opening develops into the mouth, the tube itself will become the digestive tract and the last opening creates the anus. The head starts forming around the mouth to control the organism and four small appendages will grow into legs and arms for future mobility and food procurement. A skeleton keeps the organism rigid and the entire being is wired up with nerves, blood vessels and a policing immune system to protect it. To procreate, a sex organ (vagina in females) needs to be installed somewhere in the confines of this creature. The arms and legs are so specialized with bones, muscles, tendons, and ligaments, there is no room. The skull, which is crammed with a central processing unit in a thick boney housing and containing four of the five senses is also at a loss for space. The upper torso, protected by the rib cage, houses the heart and lungs which requires extra space for expansion and contraction and can ill afford competition from a penetrating male member. Finally we get to the lower torso which is perfect. It is occupied by mostly digestive organs that are pliable and is missing the front rib cage that will allow for expansion of a growing fetus. The vagina could be placed anywhere in the front, because the back has a partial rib cage and a stout structural vertebrae and hip assembly. The most obvious location would be where the navel is now. A vagina could be placed there, and the navel relocated to the forehead, which would have generated a billion-dollar industry for decorating it. But nature is clever and placed it at the bottom of the torso, so that entry can take place from either the front or rear, thereby doubling the chance of conception, instead of just a frontal entry. That is why the vagina is located right next to the anus, it was the absolute last resort.