Hmm, don't shoot if i'm wrong.
I think it is natural that human like to look at pretty/beautiful things, aesthetic need. Personally i don't bother at all when my bf look at those pretty girls or sexy aunties out there because as a female i also look and admire those pretty creature and i also look at good looking guys.
Yes, sometimes they might be drooling at those long cellulite free legs/cleavage and might even think how does it feel to touch those but it all depends on that guy's personality whether he will act like a horny dog and cheat.
An ex of mine once told me this ' When all the lustiness gone, the only thing left is the girl's personality. What good will it be if she's sexy but we got nothing to say/ in common when out of the bedroom?'

Well. girls pls believe in yourself. If that guy dumped you just because he spotted another hot thing, then you should be glad that you are free from a dog like that.