Carin Bondar wonders whether the act started with arthropods. Among many examples:
Mormon cricket (Anabrus simplex) males provide a nutritious meal to their female partners during copulation which provides her with direct fitness benefits outside of the sexual act itself. Studies have shown that in times of food shortage females actively compete for access to males (and thus to the food benefits that come along with doing the deed). When food is not a scarce comodity, such competition does not take place.
But from a male perspective, the analogy doesn't quite work:
Male arthropods from crickets to weevils and beetles have one common goal: to fertilize as many eggs as possible with their own sperm. Their active participation in repeated copulations with females that may be only interested in the fringe benefits result from the possibility (however remote) that his sperm will sire the next generation. Human males that actively solicit sex in exchange for resources they provide (e.g. cash in most cases) are pretty much the opposite of that. In our species, sex that is acquired in exchange for goods (prostitution) is for recreational purposes only… .
The Dish 