In the book Sperm Wars, one of the main points is that "a woman is far more likely to conceive through a casual fling than through sex with her regular partner" and:
Secondly, and perhaps this is surprising, a woman is more likely to have routine sex in the two weeks after she has ovulated, when she cannot conceive, than in the two or so weeks before, when she might conceive. The difference can be detected statistically, but is too slight to be noticeable to either the man or the woman. This subtle change in behaviour is not due to a conscious decision by couples to avoid conception. Women using reliable contraceptives, including the pill, show the same slight changes in behaviour. So, too, do other primates. The subtle change in a female's willingness to have sex with her partner at different stages of her cycle is hormonal, not cerebral.
Some of the claims in this book make a lot of sense, while others are pretty dubious. I suspected this was one of the more dubious ones.
But now that I'm in an open relationship, and know everything that my girlfriend is doing with other guys, this seems very obviously true. Much more so than his "subtle change". When she's near period time, she's very "domestic" (her word) and monogamous and thinking exclusively about me. When she's near ovulation she's preoccupied with other boys, reestablishing contact with ones she hasn't talked to in a while, going on dates and sleepovers, etc. She says it's a very obvious change, too. Does anyone else experience this change in behavior?