The prejudice – the pre-judgment – comes in the elision from "some Muslims … ", "some gays … ", "some Catholics … ", "some Irish … " or whatever to the noun on its own: "Muslims are"; "Irish do"; "Catholics are".
I think this is simply a result of the way that nouns work. When we attach meaning to new words, it is rather like filling out a long form in which there are default entries for all the fields that we filled in last time. Whatever we do not consciously change is carried over from the most similar previous idea.
.... Still, I don't think it's an unworkably pious suggestion to follow the lead of our style book and check, every time you say "Muslim" (or "Christian", or "atheist") whether the sentence works just as well if you change the noun to an adjective – "Muslim people" – and only say "Muslim" if you can't hear any difference. [Guardian Cif] Read more
