Evolution has developed numerous interesting behaviors in us homo sapiens. The "flight or fight" response to danger, our love of fatty, salty and sweet foods, bipedal mobility so we can use our hands better.
But one behavior that causes strain in relationships is the activity of men's (heterosexual, that is) visual cortex (portion of the brain that takes in stimuli through the optic nerve) in the presence of an "attractive" women .
Brain scans show that men will automatically respond to attarctive women by turning towards them, without consciousness! An important ability during those prehistoric times when the preservation of the species was enhanced by incresing the size of the tribe/clan/community.
Nowadays it creates a lot of conflict when a woman mate/partner/spouse witnesses this automatic response and believes it is a statement by the man that he prefers that othe woman to her. This ain't necessarily so! he may just be unconsciously responding from a visual cortex doing it's job.
Unless he starts to drool, goes out of his way to stare, or make crude suggestive remarks, he probably isn't saying anything about his current partner.
In other words, it probably isn't personal (to either of the women), so don't take it personally.