Below, I posted on the supposed increase in female infidelity (Steve Sailer expresses skepticism, which I share, though I think some of the difference might be less social stigma resulting in more open reporting from females). Here is a website I found that might shed light on the Newsweek article:

Buss & Shackelford (1997) investigated a number of mate retention tactics used by married couples. They found that for men, the number of acts of mate retention was positively correlated with their partner’s youth and physical attractiveness. In fact, the peak levels of mate retention also correlated to the peak reproductive years for women in general.

Here is fertility as a function of age:



Finally, readers might find this long piece about male responses to the possibility of infidelity interesting (it takes a cross-cultural anthroplogical view seasoned with evolutionary psychology).