Why do men perve at other women if they already have their own girl?

Okay men c'mon - I really would love to know what it is with men that perve at other chicks if you already have a girlfriend..with out sounding like I love myself I would consider myself to be pretty good looking, my man has been asked over and over in our relationship of 15 years how he got a girlfriend that looked like me, yet he perves at other girls all the time, of course he does this discreetly when I am with him but I guess it always gets to me, makes me feel like I am having to compete..Is this silly or wot? I don't perve at other guys as I don't feel that I need to I have everything I want in my man..So this is for all the men out there who do the same thing, and serious answers please - Why do you do it???? Should I feel threatened by it? Does it mean I am not good enough or is it just admiration for how gorgeous a woman can look? If so then tell me also do you realise how hurt it makes us gals feel....I really would like the insight on to how you men think when you do it...