It depends on what you mean by 'attracted' and 'new girls'. If you mean men checking out strangers when he has a girlfriend, well that's human nature. Women do that too.
However, if you mean men straying from their girlfriends for new girls they meet, well.. that's a person-to-person thing. That could be due to lots of things. Insecurity, loneliness (remember, not all girlfriends are 'awesome'), the thrill they get from the danger of getting caught... lots of reasons.
Not sure which you meant :)