It has been suggested that about half of all marriages are 'affected' by infidelity. People take a partner's infidelities seriously, so this number seems high. There is clearly a very high risk in infidelity (or at least a high risk if caught), so the urge to do it must be very strong.

Selfish Gene Theory tells us why the urge is so strong, and why female infidelity and male infidelity are motivated by different things, though producing more descendants is the common factor.

We see from this that there are fundamental reasons for conflict between the sexes. Male infidelity is a problem, but female infidelity is the real killer. While male infidelity should be discouraged by the female, it should be tolerated unless it is causing serious problems. Female infidelity should not be tolerated by the male - there is genetically no value in raising someone else's offspring.

This is not to morally justify infidelity on the part of either sex, but it explains why we tend to see certain behaviors, and differences in behaviors between the genders.