I heard Lenin's siren call and have returned to go on about evolution again.
Fitness (what most people are talking about when they say something is 'natural behaviour' or 'design') is measured in inclusive fitness. Basically, the evolutionary fitness of an individual is based on how many of their genes are passed on to members of the next generation who themselves are capable of reproduction. So, if I have two children my inclusive fitness is the same as that of someone who has four nieces or nephews - the amount of genetic material passed on will be roughly the same.
So, given that in the environment of evolutionary adaptation that anatomically modern humans emerged in, there were limited resources, people had to find enough food to survive. In addition to this, humans are a kind of species that put a relatively small amount of energy into procreation but a huge amount of energy into raising young (as compared to say, a fly or a fish). So a lot of human fitness is about being able to raise the young. It takes on average 18 years for a child to become energy independent from their carers in a hunter-gatherer society.
If there are, for example, two sisters who meet two men and they both have two kids, that is four adults attempting to feed eight people. If there are two sisters and one is meets a man and one meets a woman, and only one sister has two kids, there are four adults attempting to feed six people. Obviously people tend to help their sibling's children and the children of people they are socially bonded to ( and some species, particularly those closely related to humans, form social bonds through non-procreative sexual acts). Now, if a bad year comes, the entirely straight family has a higher chance of starving to death than the family which includes the lesbian sister because there are more of them. Thus, the genes of the lesbian sister pass on to her nieces or nephews, and her lesbianism has led to high inclusive fitness.
This is then connected to Posie's point about Western women. The whole PIV issue is of greater importance to those living a more impoverished existence. The average birth spacing in hunter-gatherer societies is 4 years. One of the African languages word for starvation is literally 'first child second child.' It refers to starvation as a result of children being born closely together. A household group cannot be energetically independent unless the birth spacing is about 4 years. Now, part of that 4 years is explained by approximately the first 9 months of breastfeeding making a woman unable to conceive. But after that, there is a huge period of time in which women somehow avoid getting pregnant.
Some of this could be explained by infertility as a consequence of a restricted diet, but some of it also must be explained by people choosing to do things other than have PIV sex. It would seem then quite natural for want of a better word, for women to either turn to other women for sex and only turn to heterosexual sex for procreation, or for women to spend a lot of their time wanting to have sex with men in ways unconnected to procreation. Because if our early ancestors felt an overwhelming urge to have PIV sex all the time, they would have died of starvation and none of use would be here.