This thread reminds me of a recent poll, about what facets of cheating bothered people most. Interestingly, it found that generally, men were most worried about sexual infidelity, and women about emotional ties.
Most of our core attitudes about romance and sex seem to be rooted in reproduction, and reproductive strategies. And men and women have different risks when a mono partner cheats. The big risk for the woman is emotional ~ he might leave her to raise their kids alone, if he loves the other woman. The big risk for the man is sexual ~ his partner might get pregnant with another man's baby, and he'd be spending his energies raising somebody else's (biological) child. Our jealousies often reflect these things. Polyamorous folks are following other reproductive strategies, but some of these behaviors and feelings still seem to affect us.
Personally, I'm with Ariakas... I seldom feel jealousy unless I'm feeling insecure about a relationship, or about myself.