Do you agree smoking or dropping a wrapping, done only once, constitutes polluting? The thing is, you are considering only the source as an individual, not as part of a community where everyone behaves the same. Where individual rights make sense (smoking, littering, socializing), cummulative effect causes damage exceeding the pleasure of the individual relief. Consider the balance between individual and collective rights.

Yes, but the nature of pollution is doing that, even once. The nature of harassment is continuing to do something after someone has expressed that it's unwanted. Now, unless it's something explicit where common sense says the typical person would not find that desirable, then you can't know until after the first thing said and a reaction is given, if it's unwanted or not, since typically the person is walking past and you have no opportunity for any body language reading (not that most people are good at that anyway).

And as you say individual rights vs collective rights, I'd say the right to be able to say anything you'd like trumps the fact that it might bother someone (especially if it's unknowingly).

With the cummulative effect of every person trying to approach them, including some of the scary ones empowered by the moderate ones? Remember the moderates empower the radicals, this is true for every movement.

If one person does something, it's their problem. If everyone is doing something, it's your problem. It's along the lines of if everywhere you go smells like shit, check your shoes. If a single person is being constantly catcalled, there's probably a reason for it at that point. If it's only a few, then there's probably not. Keep in mind I disagree with the whole anti-cat calling movement completely. It's just words. I have to deal with a lot of people saying things I don't want to hear. I deal with it. In almost every case, these comments are transient, and the person is never seen again. (And if it's not the case, then it's possibly stalking, NOT cat calling).

You have unrealistic expectations from posters. No-one expects a poster to single-handedly change attitudes but be a part of a campaign that includes posters, articles in media, entries in social networks like youtube, twitter, reddit (this discussion), blogs, etc. that cause an incremental effect over time. Creating awareness is a start. You are claiming the start is not the end and this somehow negates the whole intention. I contest this.

You misunderstand. The posters themselves are offensive. The only people they will reach are the people who wouldn't do it to begin with, so it's like someone telling you to do something you're already doing under the assumption that you don't know better.

The people it's intended to reach, really don't care.

An analogy to this would be the whole "meat is murder" campaign. I see those posters from time to time. I don't intend to stop eating meat, and the poster doesn't in any way lead to that.

Something to note: When I refer to cat calling, if it's something explicit and suggestive (soliciting), or implying use of force (threatening), they already have names, and are crimes in many cases. I'm not, including those in any point I'm making.