Less fit by the standards of natural selection, anyhow:
The following table shows the average number of children women have birthed by the number of male sexual partners they have had since the age of 18*. Like men, women who have had only one partner are the most fecund.
Whereas monogamous women who have only had one lifetime partner averages 2.29 children, the average US woman with nine partners averages 1.46 children. The big dropoff appears to be between six and seven partners, which I find somewhat interesting because I recall an old numerical definition of promiscuity from one of my school health textbooks as being six or more partners.
Religion would appear to account for most, but not all, of this effect, as regular church attendance is correlated with an increasae of about .7 kids/woman.