From Wikipedia: "Sexism, a term coined in the mid-20th century, is the belief or attitude that one gender or sex is inferior to, less competent, or less valuable than the other. It can also refer to hatred of, or prejudice towards, either sex as a whole (see misogyny and misandry), or the application of stereotypes of masculinity in relation to men, or of femininity in relation to women. It is also called male and female chauvinism. Sexist is the adjective form of the noun sexism. Sex discrimination is discrimination on the basis of sex or gender."
Restricting an endowed professorship on the basis of any criterion is not sexist; it is discriminatory. And, frankly, I think it's totally justified: women are drastically underrepresented in the field, due to a wide variety of subtle discouragements that mostly fall under the broad label of institutional sexism.
The metaphor my partner uses is of a seesaw with one side on the ground. If you want to bring it back into balance, you have to make a deliberate change to the system by either removing mass from the side that's down, or adding mass to the side that's up. Continuing to pile mass on in the same proportions as before is, well, take your pick between "if we do not change our direction, we will end up where we are going," or "the definition of insanity."