"Static" is definitely the wrong word. I think you are looking for "immutable", which is mentioned on
https://en.wikipedia.org/wiki/Suspect_classification . However, note that "the [Supreme] Court has not declared that any particular set of criteria are either necessary or sufficient to qualify" as a suspect classification.
In any case, the idea of a "protected class" is not "based on being something you can't change about yourself".
That you may want it to be something else is irrelevant. What you believe is wrong and absurd (using your terms), based on the decades of established law and court cases on the topic.
You wrote: "mere opinions protected from critics on the ground of fighting discrimination. It should always be possible to say to someone that what they believe is wrong or simply absurd"
It is (almost) always possible. The question is, should that expression of an opinion be free from negative consequences? The answer is, for the most part, "no".
Furthermore, having "mere opinions" is one thing, while expressing those opinions are another. You might be of the opinion that women are incompetent and should not be hired, and if a women is hired you might express that general opinion to her.
However, as https://www.eeoc.gov/laws/types/sex.cfm points out, in the workplace "it is illegal to harass a woman by making offensive comments about women in general."
(Note: "Unless the conduct is quite severe, a single incident or isolated incidents of offensive sexual conduct or remarks generally do not create an abusive environment. As the Court noted in Vinson, "mere utterance of an ethnic or racial epithet which engenders offensive feelings in an employee would not affect the conditions of employment to a sufficiently significant degree to violate Title VII." ... A "hostile environment" claim generally requires a showing of a pattern of offensive conduct". https://www.eeoc.gov/policy/docs/currentissues.html )
There's plenty of examples in the EEOC guidelines to help understand when something goes beyond "mere opinions" and into the realm where the courts may decide that a given workplace environment is "hostile or offensive".