What is the point of relationships when attraction exists?

I was thinking of this earlier as I watching an episode of Misfits, now if you are aware, profanity in European tv has risen over the past few years and this show although has an interesting plot, bears quite a few sex scenes, now me, my sister and her bf were watching this and I left the room. A few hours later my sister's bf left and my sister was telling me how she felt it was odd for men to be attracted to other women despite being in a relationship and she broke down in tears because she felt her esteem was hit by this attraction or whatever. Now obviously if my sister's bf did comment aloud he's a prick, but what are your opinions and how can i make my sister feel better?