I agree with pron. You wrote "Women are simply less productive, due to exiting the workforce. There is neither distaste nor bias."
The question is, why do they leave the workforce? If it's because of a self-reinforcing belief that they less valuable solely because they are women, then it's sexist. If such a belief is widespread and entrenched, it means the culture is sexist.
Eg, if it's because women want to take a year off after the first child, then one interpretation is that it's a choice. But the question is, why don't the men want to take a year off? Is it due to cultural expectations? Is it that mean get paid more because of a built-in expectation that they will be the ones supporting the family? If so, there's a sexist cultural component to the decision.
Hence the more feminist cultures have parental leave. Sweden, where I live, has paid leave for both parents, and some of the leave can only be taken by the mother and some only by the father. That doesn't mean that the leave is split 50-50. More women take time off than men. One interpretation is that Swedish culture is still sexist. It does bring a smile to my face to see the number of fathers out with their kids, compared to what I'm used to from the US.
Or as another example, consider one of the famous cases of sexual discrimination at work. Quoting from the EEOC, "In Robinson v. Jacksonville Shipyards, Inc., a shipyard company employed a female welder who was continually subjected to nude and partially nude pictures posted by her male co-workers. The men posted these pictures not only in common areas, but also in places where the victim would have to encounter them, including her tool box. The men referred to the victim as "baby," "sugar," "momma," and "dear." In addition, the men wrote obscene graffiti directed at the victim all over the plant. The men also made numerous suggestive and offensive remarks to the victim concerning her body and the pictures posted on the walls. The victim complained about this atmosphere of harassment on a number of occasions, but the company's supervisory personnel provided little or no assistance. The court found this conduct violated Title VII because the plaintiff belonged to a protected category, was subject to unwelcome sexual harassment, the harassment was based on sex, it affected a term or condition of her employment, and the employer knew or should have known about the harassment and failed to take remedial action."
If most women exit the workforce in order to avoid harassment caused solely because of their gender, then it's due to sexist reasons.
Hence why pron and I disagree with your assessment that "Normally the term [sexist] implies some sort of (non-productivity based) distaste for women or bias against believing they will be productive." The sexual harassment in Robinson v. Jacksonville Shipyards, Inc. had little to do with her being productive or not, but because she was a woman. I have little doubt that being the most productive person in the group would still have resulted in sexist behavior, and in any case if she were not appropriately productive she would not have been promoted to first-class welder.
Pron wrote "Sexism is any gender based discrimination that is not 100% provably biologically justified." Such a definition is in close agreement with the EEOC definition of discrimination on the basis of sex in the workplace. (They aren't the same; I assume that "100% provably biologically justified" is for wet nurses and surrogate mothers, while the EEOC allows a wider range of 'bona fide' reasons to discriminate on the basis of sex, like only hiring men as Chippendale dancers.)
Your definition is not, which leads to the strange observation that you must think that some forms of sexual discrimination which are due to the victim's gender are not actually sexist.