Research conducted by the Universities of Exeter and Liverpool concludes that females that have multiple mates reduce the risk of producing a brood of offspring without males and this could prevent the species becoming extinct.
The study into the long term effects of polyandry (multiple mates, a common phenomenon in the animal kingdom) was carried out on the fruitfly, Drosophila pseudoobscura, and covering fifteen generations.
You can read all about this interesting theory here.