Let me be clear about something, treating fat people badly is disgusting. You shouldn't treat anyone badly, and no one should feel unwelcome in places, especially common places like the gym where people feel unwelcome or unsafe, everyone has a place there.
However, expecting society to conform to you, isn't something that anyone should expect.
You will not fit in every clothing brand. I am a small person, and often I can't find clothes small enough to fit me due to things like inconsistent sizing. Often stores will carry up to a 3X and will not carry a size smaller than a S or XS, and I simply can't fit in the clothing item, which sucks, but then I go find somewhere else.
Not every place will cater to you, and that's okay.
Different things are meant for different people, and that doesn't make it fat-phobia, and I can hold this belief while still believing that of course fat people shouldn't be harassed because no one should. Those beliefs can coexist and it is ridiculous and manipulative to present the idea that they can't.