Honestly, I'd say you can think whatever you want. If you get turned on by something in public, what difference does it make? If you're not actually going to take any actions to involve another person, and it's all in your head, then great. Welcome to being human.
For example, I like tits. Many women happen to have tits. I do look at them from time to time and let my imagination wander (though I do not stare). This does not affect anyone else and is a natural part of humans being sexual creatures.
On the other hand, if you are going to do something that directly involves another person, the main thing is to ask yourself the following question first:
"If the person I'm involving knew that there was a fetishistic motivation behind it, would they still want to do it?"
If the answer is "no", then don't do it.
To add some additional guidance, some of the quick questions you can ask yourself to determine whether action is ethical are:
"Would I be okay if what I'm doing were on the front page of the news?"
"Would I be okay if my family members knew what I was doing?"
If the answer to either of those questions is "no", then what you're doing is likely unethical and you should refrain from doing it.
MessyDareGirl said: I think what this guy's wanting is closer to your "no-no" example, though. They're not just happening to WAM in public, or happening to be getting off to it in public, they are getting off on other people watching them. And people haven't consented to that
Oh, you're absolutely right here. What's being asked for is definitely unethical. The question then becomes "is it legal?" which it might be.
So can you do it without legal repercussions? Maybe. Will people think you're a shithead for doing it? Definitely.
(Note that this is legal advice. I'm not a lawyer and have no clue whether it's actually legal or not. Don't blame me if you get arrested for forcing your fetish on unwitting bystanders.)