come across as believing that someone should be treated the exact same way whether they're in a business suit or a bathing suit, regardless of whether they're in an office or in a pool.

That's a valid deduction.

I don't see what "first impressions" have to do with this discussion. We're talking about cosplay, not a professional meeting.

you're pissed off about people harassing others and you associate how they dress with harassment.

Yes? When the harassment is directly a response to how they dress, that would make sense. In this scenario, we're talking about someone who dressed normally and was harassed to dress less so. It's ludicrous, and the fact that anyone would defend it through proxy or strawman debates is horrifying.

People at conventions seem to feel that the way someone dresses is related to the level of permission they are given to harass, verbally or physically. The number of times female cosplayers I know have been grabbed, photographed without their immediate knowledge, or had disgusting remarks in person or on line made towards them because of their costumes speaks to this issue. The fact that, when this is brought up, some people promptly jump in to divert the conversation and claim that it happens to male cosplayers, also, suggests it is as much about attire as it is gender.

He knows both of those already and he does not care.

Yep. This is the worst case scenario, someone who understands and does it despite knowing better. What this means is that the community needs to oust and denounce that activity, so that, if it happens despite someone's better judgement, they will be treated as they deserve, with rejection and in support of the victim.

Victim blaming and shaming isn't really alright. For reference.