I would think that if I paid for a first-class ticket (or any other ticket for that matter), I would be able to dress as I want. If a t-shirt and shorts is what I feel like wearing, then so be it.

I admire your dressing-up for travel. I try to do the same.

But I can see the other person's point of view as well.

I have seen people, travelling or in the office or at a restaurant or otherwise, I myself consider dressed inappropriately, but I don't find them offensive per se. At least not until their private bits are starting to show.

Why do you take this issue much to heart?