Keep your eyes in your head

There's currently a debate over at Thus Spake Zuska about the mysterious grey area where a glance at a breast becomes an unwelcome stare. So far the main talking points seem to be:

-It's biological, people can't help looking at an attractive person
-Humans are sexual creatures and it's unreasonable of society or anyone to expect people to leave their personal foibles and bad habits at home
-People shouldn't be so easily offended, AKA Some women like it
-People should cover up in burkas if they are so incredibly gorgeous

Zuska has done a wonderful job of dispelling the last of these: In countries where women do wear burkas, they get much more harassment than they do in countries with more liberal clothing policies. As to the other three...

It's biological, people can't help looking at an attractive person
Lots of things are biological, and yet we train ourselves out of them in polite company. Adults are potty-trained, eat food with silverware, wear clothing outdoors and in mixed company, and bathe themselves, all means of overcoming natural biological things that are offensive to polite society. You don't get a pass on those, so why should I give you a pass on where you rest your eyes? Practice the good habit of looking someone in the face, always, until you get it right. If prolonged eye contact feels uncomfortable, look at a spot beside their ear, gaze into a notebook or coffee cup, or look to one side while you talk. You can help it. If you can master the art of not picking a wedgie in public, then you can learn to look at someone politely. The added bonus of this good habit is that people truly feel like you are listening intently to them, even if you're only contemplating the pimple on their nose. Male or female, when someone feels like they are being listened to intently, they usually feel a lot better about your conversation even if they did all the talking.

Humans are sexual creatures and it's unreasonable of society or anyone to expect people to leave their personal foibles and bad habits at home
It's not unreasonable. It's just not, and there's an end of it. You don't pick your nose, pick your butt, squeeze pimples or masturbate in public, and those are perfectly reasonable expectations. If you want to know someone better, ask them out for coffee or beer. If you absolutely must know if they are generally attractive on the outside, use the miracle of peripheral vision or look from a respectable distance such that no one could tell if you were checking out boobs or merely trying to recognize the person. It's just not that hard. It's bad for your career to flirt at work anyway.

People shouldn't be so easily offended,
Some people are offended. If you offend people, it's very easy for you to be labeled That Guy in your social circle, in which case you will have to make some friends outside your circle because no one wants to hang out with That Guy, and it's so much easier to cut That Guy out of activities than it is to deal with angry, offended people making the occasion uncomfortable for everyone. Arguing that they should not be offended is missing the point entirely: They are offended no matter what you intended. Your choices consist of finding a new workplace, finding new friends, or apologizing for the offense and making a concerted effort not to do it again. "No one should mind my disgusting lack of socialization" is not a viable option here.
AKA Some women like it
And some don't. Yes, there are people out there who have a higher bullshit tolerance than others, bless their hearts. I'm sure not one of them, and I'm not alone. Some people are into all sorts of fetishes and bad habits that others find perfectly repulsive. The difference is, they know that their bad habit/fetish is not everyone's cup of tea, and keep that to themselves in mixed company. This is why the good Lord created the VCR, the DVD player and the internet, so you can keep your voyeurism in the privacy of your own home.