Why do guys check out other women when in a committed relationship? ?

I need a lot of reasons to why? It's not normal for me but rather used to it by now. We have been together for over years now he still shows feelings for me treats me like I'm everything and loves me for me. I'm not insecure but makes me jealous and mad too the point where I feel a bit down about myself. I treat him good and take care of him on my part. He shows he's highly attractive to me. He was honest a few times about why he liked a few random girls we seen when we where at the mall, gas station etc.. he described her features and what he liked and he says it like it's not a bad thing, I just don't get it. But if I do it just once back to him he assumes The worst? it's hard sometimes to cope w this depending on the issue. I tell him how I feel about it. Can't help it sometimes but to get really mad jealous w him. So sometimes I try taking him to a strip club or have a lap dance or even have something but he says only w me he needs in his life no other. So I tell him okay I'll participate w another girl to satisfy his needs but completely denies it. The fact that I feel like it's behind my back makes me angry. Guys w committed relationships , id perfer you guys to answer this. Not guys with no value pls and girls how do you cope with this?
Why do guys check out other women when in a committed relationship? ?
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