Cheating happens. Men are more likely to push for it with a fertile woman, and woman are more likely to desire it when fertile.in the latest Quarterly Review of Biology:book review
In Study 1, the scent of women near peak levels of fertility heightened the men’s implicit accessibility to sexual concepts. Study 2 demonstrated that, among men who reported being particularly sensitive to odors, scent cues of fertility triggered heightened perceptions of women’s sexual arousal. Study 3 revealed that in a face-to-face interaction, cues of fertility increased men’s tendency to make risky decisions and to behaviorally mimic a female partner. …
Whereas women may have been selected to suppress cues of ovulation in order to sustain men’s commitment, men have been selected to identify fertility cues in order to enhance a short-term mating endeavor’s probability of reproductive success. … It is unlikely that all indicators of fertility could be suppressed, because some detectable shifts in hormones are needed to facilitate ovulation. Consequently, men must rely on fairly subtle cues (e.g., changes in scent and skin tone) associated with those hormonal shifts to help them respond adaptively to women’s changing levels of fertility.
The Evolutionary Biology of Human Female Sexuality.  … Thornhill and Gangestad argue that [human] women possess two distinct evolved sexualities. One is the “extended sexuality” that women engage in when conception is impossible; the other they call “estrous” sexuality. The former functions to elicit “material beneﬁts” from males, the latter to acquire “good genes” for offspring, and in keeping with these distinct functions, candidate male partners are evaluated on distinct criteria in the two contexts.