In visual culture studies we learn that the practice of looking involves an interplay between gaze and object. "Object" is a passive term, and we use it to denote a negative position in the viewer-viewee relationship. Traditionally in visual culture, the objects of dominant gazes are women and visible minorities. Those images that are historically subversive are those that work contrary to the male dominant gaze - VISC 101:

Manet:

http://en.wikipedia.org/wiki/Image:Manet%2C_Edouard_-_Olympia%2C_1863.jpg

Olympia returns the male gaze to the horror of the french salons. Here a prostitute takes the institutional female pose and looks directly at the viewer as he ogles her body.

A bar at the Folies-Bergere

The man in the top-hat (upper right) propositions the waitress for sex and she defiantly refuses by returning his gaze (the viewer is the man reflected in the mirror).

A contemporary example, Jeff Wall: http://courses.washington.edu/hypertxt/cgi-bin/12.228.185.206/html/viewer/jeffbar2_400.jpg

The reason I bring this up is because out practices of looking are not independent of those we have been institutionalized into. There is a pre-existing culture of how we observe the marked versus the unmarked. David is right to make the distinction between female and gay male objectification. The objectification of men by other men is a reversal of the male gaze which denies the centuries old European tradition and in turn makes people tied to it very uncomfortable. (Thus the: "I don't care if he's gay as long as he doesn't his on me" syndrome.)

Innocent or not the act of looking at others is still part of that European tradition of domination and subjectivity. Women, consciously or not, are very much aware of when they are being subjugated by the male gaze. Some believe that by actively participating in this subjugation they can become empowered (see hotmail-douchebag's oh-so-sensitive email or "Female Chauvinist Pigs" by Ariel Levy). Others (myself included) believe that this could be the case if men were equally treated on our public streets (which they ain't).

The Line: Single people use body language (including looking) to alert other single people that they are willing and available. I don't say this lightly. I find that generally we employ this body language only to those we find to be suitable matches to ourselves - as deluded as they may be at times. As good-looking as some club-going greasy-haired loser eating a street dog on my corner may be I probably won't strut my stuff in front of him whereas I might with a bearded indie-rocker on Queen west. Par example.

That being said - if you're not on the market it's not okay to make eye contact with girls you find attractive on the street. Think of it this way - if she were to catch you eye and come up to you and say "Let's go for it" you would feel very sheepish responding with "actually, I'm in a relationship".