But which is cause and which is effect? Are people more promiscuous because they are more attractive to the opposite sex (therefore more opportunities, one might think), or are people more attracted to those who might be more available?
(By the way, a man or woman may have a rational preference for partners who are capable of commitment, but sexual attraction doesn't always coincide with rational preference. And in any case, the only real proof that a potential partner is capable of commitment is that they are already committed - to someone else.)
Interestingly, the research looked not at behaviour (how much casual sex do people actually have) but preferences (are people inclined towards casual sex or longer-term commitment). The mere fact that a person wants casual sex and/or longer-term commitment doesn't mean that person is attractive or otherwise together enough to fulfil these wants.
But there are two processes that might correct this imbalance over time. The first is endogenous preference formation - people change their preferences according to their experiences. Jon Elster, a sociologist with an excellent appreciation for poetry and myth, wrote a book about this phenomenon called Sour Grapes. See also Sour Grapes, Sweet Lemons, and the Anticipatory Rationalization of the Status Quo (pdf).
The other process is evolutionary. Genes that produce people who are unattractive to the opposite sex, and genes that are associated with reproductively unsuccessful preferences, will be disadvantaged. In terms of reproduction, of course, both casual sex and longer-term commitment may be successful strategies; promiscuous people don't necessarily have more offspring, and many avoid having children altogether, while I guess most children are born into relationships that were thought at the time (by at least one partner) to be reasonably long-term.