In most cases, when men watch porn, it has no consequences whatsoever for their relationships with girlfriends or wives. Most men watch porn now and then to relax and relieve stress quickly, without becoming emotionally engaged with the process in the video.
In such cases, women should not worry at all that their boyfriend watches porn. The girl should assess how often does he watch porn, and how often they actually have sex. If they have sex more often than he watches porn, everything is totally ok.