Summarize the following:
It will do neither of you any good to get angry right away. It is likely to make him more defensive. At that point, he may start blaming you for whatever arguing you do next, rather than considering ways to change it. Sometimes your husband is only momentarily distracted by what he sees. A gentle reminder that you are still there, like touching his arm, can snap his attention back to you. It can also be a subtle way to get back his attention while not openly stating that you saw him looking. Just because your husband is looking at a pretty girl doesn’t mean he doesn’t find you attractive. Saying so out loud to him lets your husband know that you saw him, but that you still want him talking to you. A friendly reminder like this can remind him of what’s important, while also having a little bit of fun.  Something like, “That girls looks good in that dress, doesn’t she?” is a good way to acknowledge what is happening and draw your husband’s attention back to you. It is okay to acknowledge that some people are just really good looking, or dressed well. Openly communicating about anything is the sign of a healthy relationship, and your ability to speak positively about another woman’s attractiveness will help him feel more comfortable with you.

Summary:
Remain calm. Use non-verbal cues to get his attention. Acknowledge and laugh it off.