A study by David Buss asked men and women to imagine which two forms of infidelity : a) their mates were having sexual intercourse b) their mates were forming emotional commitment.

It showed that males showed a stronger negative reaction to physical infidelity, whereas females reacted more strongly to the thought of emotional infidelity. And here's the evolutionary explanation:

Males, always uncertain about who fathered their children, have evolved a greater concern for a mate's sexual infidelity. Males who did not show sexual jealousy risked raising another man's children, thus losing in the genetic struggle.

Females, on the other hand, know who gave birth to their children, however because of a female's reproductive success depends on a male's resources, females have evolved a greater concern that their mate will be attached to another woman and thus divert his attention away from raising the children.

And the responses to this thread prove exactly that :D

I'm not always right, but I'm never wrong :P - Garfield