I heard on Radio 4 (think it was women's hour but can't find the listen again programme now - any posts?) that women will try to find an acceptable parter to achieve security and social status. Whether or not that partner is acceptable socially, they have a propensity to seek genetic material for their children from a healthy, physically attractive male. If the person they're living with doesn't fit the bill, they can seek it someone else.
So...seems to me that those blokes who work long hours, earn loads, but take no exercise and don't do much about their appearance had better watch his missus. 5% of people don't have the father they think they have .
P.S. With men it's, um, less complex apparently.