OK - first things first -
Men are genetically programmed to gauge the suitability of prospective females. They look at their breasts - suitable for feeding, and their hips - suitable for child-bearing - that is all hard-wired in but ...
We are not cavemen any more, and a degree of simple mental over-riding is required to be polite. I do look at women, but I would never be so obvious that my wife would see it. If I am out with her and a pretty girl walks past, I would not dream of following her with my eyes in the way I would if I was alone.
Your boyfriend can't realise that this upsets you, and in view of your depth of feeling, I would certainly tell him and I would expect him to curb his instincts from that day on.
I would say that it is entirely an instinctive thing - I gawp at gorgeous young women all the time, although I know that they certainly would have no interest what ever in someone like me.
Looking is just that - looking - it doesn't imply that action or even anything more than simply admiring is going to follow as a result.
That said, if it upsets you, it is disrespectful to you, and he should be advised, and stop now.
Hope this helps.