Umm...can't people look at whatever they want? Or are we talking about real-life or online flirting? I'm confused...but as far as looking goes:
My husband and I look at other people (doesn't everyone?), and when we're together we comment on them, for instance I might say "There's one that you'll think is cute!" but it has nothing to do with wanting to have contact with that person or with having an affair. If that's something that I worried about, I would think that's a sign that there is something wrong with our relationship. At a minimum, communication would need improvement.