I think this is largely a myth.
Women, by that standard, should only be with men who are very big, strong, provider types with extremely pristine genes.
Look how many women fall in love with men who are infertile? Wouldn't they have never been attracted to that man if biology really ran their worlds?
The temptation is there for all of us, and for some of us a monogamous relationship holds no appeal, and therefore they choose to be polyamorous, no harm in that.
To say that monogamy isn't natural seems kinda...dumbass. Sorry =/
It may well be unnatural to *you*, but the majority of people I know wouldn't have it any other way, there are very few who truly enjoy be poly -- large numbers, but a small percentage =o)