It is sexist to believe one gender is inherently superior to another
Both sexes have their own biological and genetic superiorities. Males are biologically superior at peeing standing up. Females are biologically superior at growing humans. Biologically male humans are larger and stronger, females are smaller and weaker. Biological fact is not sexism.
It is sexist to force, unsolicited, someone of another gender to hear your thoughts that you regard their gender as inferior, even if you are correct. It is not however necessarily sexist to believe those thoughts.
While you could argue that one sex is better at certain task (which is true), superior means "being higher in rank, status, or quality." which is not the same. It it sexist to believe one sex is inherently better in quality.
It is morally wrong (and can be sexist) to promulgate that 'it is sexist to believe one gender is inherently superior to another'. Superiority is subjective, and is associated with some specific metric value. I reserve the right to value what I value in pursuit of what I see as my destiny in life.
Let's discuss this: A human being has a brain and a body. It's physical and mental "identity" defines it. Now, we can agree that men and women have pretty much same intelligence. But biologically, men are superior, there's no argument there. That leaves us with a 2-1 score with females losing.
It is not sexist to believe that a gender is better at a specific task. For instance, women are much more suited to space travel, and men are (seemingly, though not thoroughly tested) more suited for combat roles.