Since life is fundamentally about spreading one’s genes through reproduction, it is the primary goal of life. Rational beings may not have it their goal consciously, but in nature’s design it is the only goal.
Sex is a resource that women possess which men have to compete for (a facet of survival of the fittest game). A woman would be naturally selected by evolution to mate with a man who comes up as the best bet for the spread of their genes. That gives rise to the market mechanism in nature. And since the pursuit of sex is central to all of human life, the world of human beings can be viewed as sexual marketplace.
How market forces work in the domain of human sexuality is brilliantly explained in Sexual Economics by Roy F. Baumeister.
Also see: Unregulated sexual marketplace