Yuck! Am I understanding your post right? You think it is perfectly fine for a 50 year old man to stare at a 13 year old girl as long as he doesn't touch? And you don't see anything wrong with that because men are just human?

Sorry but that is just gross. I understand that some 13 y/o dress seductively and in some cases, guys truly can't tell how old they are. However, when a guy can tell that a girl is 13 and he continues to stare........well, I can't continue with my comments for this type of person on the Dis Boards because I would be banned for life.
Did you miss the part about it not being crystal clear that the girl is underage? If a man is looking (just looking, not leering and making rude gestures or anything like that) at what he THINKS is an adult young woman, what exactly is the harm? Dressing seductively really isn't the issue; it's more a matter of is she tall enough to drive, does she have curves, and is she not the giggly type? Any man I know would say that if she looks like she's over 18, then she's fair game for girl-watching. (And if the watcher is a teen himself, then the limit is a bit younger.)
It is natural for a parent to get defensive when he or she sees someone just looking appreciatively, but a parent KNOWS how old the girl in question really is. The stranger does not, and his guess may well be WAY off.
A woman's age is not always evident from a distance, clothing notwithstanding. I used to work with a woman who had a very good and very slim figure, wore her hair in a long straight bleached style, and tended to wear very form-fitting clothing that showed off how toned her her body is. She also has a high-pitched voice and laughs a lot. Her DH (a military officer) was always getting ragged on by strangers for being with someone who was SO much younger than he was. She was 43 when I met her, and he was 5 years her senior. People were always ready to card her in bars and restaurants until they got a good close look at her face and saw the fine lines (she is athletic, and she spends a LOT of time in the sun. Her age shows only on her face and hands.)
Also, IME, as a general rule, men tend to think that "old enough to date" is a much more flexible number than women do. When I was in my 20's, I had a rule that I wouldn't knowingly date anyone who was more than 4 years older than I was, as I found that the age gap caused too many taste differences. I remember once mentioning this a a male friend (a coworker) who was a married man in his early 40's at the time. He thought my rule was ridiculous. He told me that over 18 was over 18, and that for most men there was no such thing as too large an age gap as long as the person was of age. I thought this was interesting, so in the 20 years since then I've asked many men over 40 what their "old enough" threshhold would be if they were still single. About half of them said 18, but not ONE of them ever named a number higher than 22. Most women would think it was ridiculous for a 50 yo man to think that he could date a 22 yo woman, but a solid majority of single men that age, if given the opportunity, would take it.