One of my American friends told me that 'He is a gay' or 'He is an American' is a rather rude expression. She said that 'He is gay' or 'He is American' is better. She could not explain why but guessed that a/an in front of adjective brings about a feeling of specifying or judging. Could anyone help me on this?
A something means “reducible to that thing”. Instead of “she is a waiter”, perhaps “she is waiting tables” would be more tactful.
Similarly, “He is a gay” is reducing that person to their sexual preference, whereas “he is gay” describes his orientation in a detached, more indirect way, where we are not labeling him gay, but answering the question, what is the sexual orientation.
On the other hand, “He is an American” just is not preferred, though it is not something that would raise any red flags in a professional setting. However, nearly only foreigners use the indefinite article (or definite article “the”), and usually not in an explicitly positive tone. For example, I am imagining salty Europeans critising Americans because we don’t pay more money for food or something classist like that.
Hope that helps.
Avoiding the use of a noun to define somebody is to use people-first language.
In its "An Introductory Guide to Disability Language and Empowerment," The Syracuse University says this about people-first language:1
What is 'people-first language'? Does everyone use it?
"People-first" or "person-first" language is a way of describing disability that involves putting the word "person" or "people" before the word "disability" or the name of a disability, rather than placing the disability first and using it as an adjective. Some examples of people-first language might include saying "person with a disability," "woman with cerebral palsy," and "man with an intellectual disability." The purpose of people-first language is to promote the idea that someone's disability label is just a disability label—not the defining characteristic of the entire individual. Many guides on disability language and etiquette may likely emphasize using person-first language, except, perhaps, when discussing certain disability cultural groups that explicitly describe themselves with disability-first language. Thus, while it is generally a safe bet to use people-first language, there are members of certain disability groups in the US who prefer not to use it, such as the American Deaf community and a number of Autistic people/Autistics. The basic reason behind members of these groups' dislike for the application of people-first language to themselves is that they consider their disabilities to be inseparable parts of who they are. Using person-first language, some also argue, makes the disability into something negative, which can and should be separated from the person.
Another example is an entry from the Disability Language Style Guide (produced by the National Center on Disability and Journalism):
NCDJ Recommendation: Avoid referring to an individual as a diabetic (a noun). Instead, use people-first language, stating that someone has diabetes, is living with diabetes or has been diagnosed with diabetes rather than referring to him or her as a diabetic.
With respect to the phrase a gay, people who dislike it do so because they believe that it objectifies them. They want to hear people-first language used instead: they are gay, not they are a gay.
Although I have never heard of a similar objection to the phrase an American, the argument against it would likely be framed along the same lines.
1 Note that by providing quotations about the link between disabilities and people-first language, I am in no way associating sexual preference or nationality with having a disability. It's just that disabilities are the common context for a discussion of people-first language.