What do you suggest as an alternative? Just have the women come into their field of vision and let whatever happens happen?
Why can't women be in the field of vision? Men need to learn how to control themselves. Men should not be walking around with the intention of oogling whatever is around. I've been secular and now religious. I know there are two different kinds of walking around. And going around with that intention or not - is definitely in the person's control. If a guy is walking around "looking for material," (so to speak) a pair of glasses isn't going to help them anyway, they are being negligent and of course that leads to sin and at least sinful thoughts.
If you are leaving bet midrash and walking home and there happens to be a lady passing you on the sidewalk, so what? On shabbos, when I'm alone and a woman by herself passes me by I greet them "good shabbos." Same as if it was a man. But if you are walking around waiting for the moment any lady will pass or come near so you'll have the chance to check out her goods, of course you're going to stare/look in a bad way. I just don't get why the fact that women are around means men have to sin. That's not the case.
If you are talking about women walking around in bikinis, then that makes sense, but most places on earth that is not happening.
And if a woman just "in the field of vision" is a problem, then how can someone do business with a woman, or shop in a supermarket, or have to TALK to a woman (like a shadchan for their kid or friend/relative, or maybe a rebitzen to inquire about their rabbi) etc etc.
See, it starts with walking in the street, then it will go to buses, then schools, then shopping malls, then supermarket, etc. This is how this crazy stuff gets started. Because it's an easy "jump" to all those other things once it's accepted for walking on the sidewalk.