I noticed my ex got really inflamed when I told her I thought some actor from a movie was pretty.. I thought it was a little ridiculous because it was an actor in particular. But If he keeps on referring to his ex`s in that way, then he is being very, just like The Wizard said, "insensitive."
My advice is to make sure he understands that he is being out of line. He`s most likely NOT joking, unless he has very dry, sarcastic nature. I`ve actually used the old, "Oh, I was just joking..." line quite a few times before. Sometimes there is no way around it. And if he uses that, then it should be regarded as him realizing it hurt your feelings and IS actually acknowledging it. The real problem, though, is if he keeps on using it...
Just my two cents.
Watch it happen to other people, and eventually watch it happen to you.