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For those of us who really aren't all that interested in checking out chicks, amused tolerance works.
It's all about biology. They can't help it. They are wired to check out chicks, even when they're happily attached. The difference between you and the chicks they check out: you are there, accessible, familiar (which is generally a good thing), and perfectly satisfactory.
So a good-natured ribbing is perfectly fine. Make fun of him. Tell him, "It's ok. I understand it. You can't help millions of years of evolutionary biology. Just KEEP YOUR EYES ON THE ROAD, please, because I don't wnat to have to tell the police that my husband caused a fender bender because he was checking out some chick's butt."
Or be really obvious about checking out a good-looking guy and see what his reaction is.
Whatever the case, keep it light. He's probably feeling a little guilty about this. A little guilt is good. Too much will make you seem a joyless harridan.