I mean if he looks for few only seconds it might be okay (we are all attracted by beautiful things). However if he keeps on looking few times up to the point he is starring , it would probably the sign of 'instant attraction'. Not a case whether he 'gay' or not. It is about respect. Maybe he's just not that into you. You just his second best...who knows.
I personally don't like the idea about this regardless of whether a man do it or a woman does it. From the look goes to smile. From smile goes to talk. From talk goes to exchanging facebook and msn. Then what?....Magic word...."Affair"?