MadBadDaddy:
As for 'defending women's sex-based rights against sex based oppression' you'd have to explain how they are attacked by trans people's rights.
From what I understand, by using the word 'sex' and not 'gender' they implicitly put themselves in opposition to trans rights so are designed to exclude trans women, so yes that would be problematic.
I think you answer the question in your first sentence with your second sentence. If we cannot mention biological sex then we cannot mention sexism and then we cannot fight against sex-based oppression and then feminism will be toothless. Ultimately we cannot collect data on the basis of sex and we cannot organise politically to work for the issues female-bodied people share.
The initial subjugation of women, as a class, was not based on gender identity but on biological sex, especially the two major differences between male and female human beings: Only women can gestate a child and women, on average, are smaller, slower and weaker than men.
Those differences explain why women ended up in an inferior position. Repeated pregnancies, lactation and so on kept women away from the public areas where power was doled out and those things, combined with the smaller bodies, meant that women were less able to defend themselves against both men from other tribes who might have wanted access to their reproductive systems and perhaps even the men from their own tribes.
The oppression of women has in all societies been based on sex, not gender identity. Misogynistic tenets of the three major Abrahamic religions are sex-based, for instance, and all the discriminatory laws of the past were sex-based. Women were not allowed to vote on the basis of gender identity but on the basis of sex. (Note that it is really 'perceived' sex which matters there, i.e., what others assume someone's sex is, so in that sense trans women would also be affected by sex-based oppression (and obviously nonbinary female-bodied people).)
Sex discrimination in the labor market is because only female-bodied people can get pregnant. Some bigotry is based on gender stereotypes, true, but the underlying reason for the discriminatory outcomes is sex-based.
Sexual harassment and sexual assaults are overwhelmingly directed at those whom the harasser or assaulter regards as female, and it is rather rare for female-bodied people to be the ones who harass or sexually assault.
It seems that among the woke we are not allowed to have a name for the group which is likely to be subjected to sexism etc., because naming that group is transphobic. This is a terrible framing of the situation because it implies that fighting against sex-based oppression is a transphobic act.