Modern English often inaccurately uses the word jealousy to make it synonymous with envy. Please allow me a brief example:
My wife and I are at a swing dance event. I watch her dancing with someone else.Got it? Jealousy is being stirred to an appreciation of what you have. Envy is wanting what someone else has.
If I feel jealous then I think, "My wife is attractive and a good dancer! That other guy may be dancing with her now, but I get to dance with her later and take her home."
If I feel envy then I think, "Hey, that's my wife! Get your hands off her!"
Now, I have already mentioned that my wife and I both think the other is sexy, that we have physical and emotional exclusivity, and that we were abstinent until married. These three factors combine to help us feel safely secure in our relationship.
But I am sure there is more that allows us to not feel threatened by the fact that the world includes other attractive people. We even enjoy trying to put our hard-wired attraction preferences under the microscope while people-watching or looking at a catalog. (From one such discussion I learned my wife is really attracted to strong hands. So I should wrap this up soon and give her another backrub to keep my hands strong and sexy.)
Here is a more detailed recent example involving this photograph from the back cover of a college alumni magazine that was lying on the dining room table one day. Not only did we disagree about which young lady was most attractive, but I could not explain why I picked the one third from the right. Some cooperative analysis revealed that it was the combination of high cheekbones and curved eyebrows: I must be wired to prioritize a Duchenne smile in which the limbic brain engages the eyes. (In the photograph all the other young women instead demonstrate a forced "social smile".) No wonder I like making my wife laugh!
It is comfortingly sexy to know my wife is enough of a sexual creature to consider other people attractive. And it is comfortingly stable to know that attraction will produce jealousy, not envy.