OP my ex used to do this all the time.
Not just blondes but all attractive females, and it used to make me feel uncomfortable, inferior, insecure, upset etc. etc. I used to bring it up sometimes and I'd also get the 'I'm only looking' and the 'but I'm with you/I chose you' line.
We ran our own business and across the road was a catering college where you could get a fantastic cooked lunch for a couple of quid in a restaurant run by the students. Great value and really good food, so we'd go over there most days.
Lots of those serving and working in the restaurant were young attractive female students and I was sitting opposite him one day and watching him ogling these girls and I suddenly saw him in that moment for what he actually was - a pathetic middle aged letch, ogling girls who wouldn't look twice at him. It was literally a eureka moment when the scales fell from my eyes and and I saw him for the sad twat he really was.
After that it never bothered me and I'd just laugh to myself as he gawped at some lovely young thing as she breezed past and didn't so much as glance in his direction.
But I also started doing what others have suggested here and that is doing a bit of 'looking' myself. Just clocking nice looking blokes and making sure my ex saw me doing it. Also, there's an interesting thing I found and that if you 'look' at a bloke in a certain way, quite a few of them will return the look and make eye contact. I've not trying to make out I'm some raving beauty and I had men ogling me or anything like that. it may just be an automatic response and they may not even be aware of doing it, but men started looking back at me, and my ex noticed it and was not happy. I remember going to a gig once and made eye contact with this really nice looking bloke and after that he kept looking over and giving me the eye and my ex fucking hated it haha!!! It was also a massive ego boost for me, one that was so badly needed at the time after being made to feel like shit for so long by him.
He was an arsehole in loads of other ways (not just the letching) and I got rid not long after that, but dealing with that particular situation was a real turning point for me and after that I just got stronger and stronger and less inclined to tolerate his shit in other areas.
You shouldn't have to put up with this OP. At the very least its inconsiderate and disrespectful, but more than that it's insulting and demeaning and can erode self-esteem and self-worth hugely, as you are already discovering.