Why do women get jealous when their men look at other women?

The odd thing is, I like women too. But I get jealous when my man looks at naked mags, and checks out other hot chicks. Is it purely an insecurity problem that causes jealousy? Or is there some kind of evolutionary or instinctual explanation? I don't think it's intuitive (although in some cases it can definitly be because he's cheating), I have full trust in his fidelity. It just irritates me that I'm even jealous in the first place! There's no need for jealousy!