Yes, I'd agree, the internet ist a public place. For that reason, one should be careful what one puts out there. Period.
But is it that easy? It most certainly is not. What is posted on the internet is usually posted for a certain, well-defined reason. For example, a boy or girl wants to share her experience with her headgear or her new lipgloss or her iphone with people of their own age. Or they want to help others in the same situation. Or a proud mother or father simply want to share photos of their beach vacation with the world, including bikini shots of their beautiful children, who just happen to have braces. Or someone is an aspiring photographer and may want to show their best pictures, who happen to include people in braces. Someone might even get closer to the erotic side of things. None of these people want their pictures and videos used as fetish items - but that is exactly, what we, who are here on this forum, do. In other words, we use these pictures, videos, etc. for something they were never intended for. Like using a power drill to stir pizza dough. We abuse these materials.
BTW, I would agree to MikeB's theory about at least some fetishists and their social insecurities and sexual frustration. I'm not sure it's a hen and egg thing, but IMHO there is certainly a correlation.
I think Braceface2015 has a point in that you have no control over what you put out there once it IS out there. The first problem with that is the digital nature of what is online. Digital items can be hacked, stored, copied and reposted. So far, there is no real protection against that, especially on the more popular sites like Instagram, Flickr or Youtube. Copying some one else's copyrighted material shouldn't happen, of course. It's stealing. Yet it does happen several billion times everyday. The second problem is intention - what, in the study of literature, we call author's intent. As mentioned above, our sources or authors have an intended viewership in mind, as is their right. We are not that viewership, none of us here. To give a simple example, Vladimir Nabokov most certainly never intended Lolita to be read as "the story of my life" by anybody. Yet I would bet anything there are people on this forum who can fully associate with the feelings of Humbert Humbert. The third problem is, that not everybody realises that putting things online is like putting them in a newspaper. It is publishing. Only much more intensely so, since the audience is the world, not just a few buyers of the book. But since that audience is so anonymous, I don't think most people are aware of that, especially not all youngsters (but no age restriction there, or anywhere in what I write here).
For my part, I think abusing materials put out there in the way we do is inevitable. It's in the nature of something you publish. People have masturbated over Playboy Bunnies for as long as Playboy existed. Even the copying of digital property is, in a way, inevitable. Facts of online life, like it or not. However:
It is unacceptable to comment back to the authors in a way that is obviously offensive. In fact, I think it's unacceptable to comment in any sense that is not intended by who put the stuff out there. It is unacceptable to force your own deviant phatasies on someone else. Any of this is mental abuse. It is akin to physical abuse or even rape. Also, it is unacceptable to stalk. It is also unacceptable to copy material in to a context it is not intended for. It may be acceptable to comment that a photo gives you a hard-on on an erotic or pornography-related site, but not on, let's say, the models private / professional site or their account on a general public social media site.
BTW, I don't think clicking on the other videos in a channel to "even out the numbers" is correct. That is a form of camouflage to lurk behind while stalking. Let's at least be honest about it and look at the content that does interest us, sending a clear message. If the authors are uncomfortable with that and delete the content or stop posting similar content, then that is their choice and for us to accept.
Also, you people out there, if you do find yourself in a similar mindset to Humbert Humbert: It's okay, it's part of who you are and how nature made you. Just be fully aware that at least 99% of the general population don't feel the way you feel, including the person you're having a hard-on about. You will be respected as you are, no matter what, as long as you respect everybody else around you. Respect is a two-way road, and it is about empathy. Unless you're 100% sure the person opposite feels about your comment the same way you do, please remain silent. And since you cannot ever be 100% sure about that, please just remain silent. Full stop.
Just my two cent's worth on the subject... And yes, I am aware that by being here and using this forum, I, too, do not conform to the maxims I've set out above. In moments of sober reflection like now, that kind of ambiguity - call it bigotry if you like - is not an easy thing to life with.
--SC