A study of fruit flies by the Natural Environment Research Council in the U.K. has found that promiscuous females of a species may be the key to that species' survival.
The reason is that when females partner with fewer mates, the likelihood that they'll have all-female broods rises. A so-called "sex ratio distortion" (SR) chromosome exists that kills Y chromosome (male) sperm before fertilization. Female offspring carry the SR, which will be passed on to their male children until there are all-female broods born.
Short version: Do it more, widen your brood-load and reduce the risk of getting all-female broods. Species saved!
Read more about the study and polyandry here.