This might be of interest to the evo bio and game theory wannabes here: "Sex Is Always Well Worth Its Two-Fold Cost" by Alexander Feigel, Avraham Englander and Assaf Engel.
Sex is considered as an evolutionary paradox, since its positive contribution to Darwinian fitness remains unverified for some species. Defenses against unpredictable threats (parasites, fluctuating environment and deleterious mutations) are indeed significantly improved by wider genetic variability and by positive epistasis gained by sexual reproduction. The corresponding evolutionary advantages, however, do not overcome universally the barrier of the two-fold cost for sharing half of one's offspring genome with another member of the population. Here we show that sexual reproduction emerges and is maintained even when its Darwinian fitness is twice as low as the fitness of asexuals. We also show that more than two sexes (inheritance of genetic material from three or even more parents) are always evolutionary unstable. Our approach generalizes the evolutionary game theory to analyze species whose members are able to sense the sexual state of their conspecifics and to adapt their own sex consequently, either by switching or by taxis towards the highest concentration of the complementary sex. The widespread emergence and maintenance of sex follows therefore from its co-evolution with the even more widespread environmental sensing abilities.
I'm currently trying to parse the article, and on first reading could only see a disguised form of the old familiar argument about the stability of sex ratios. It still doesn't seem to answer why females don't switch to parthenogenesis and block all male advances. But maybe you can detect something I missed?