The whole reason women have ‘Womens Rights’ on top of ‘Universal Human Rights’ is because of the inherent disadvantage that comes with a female body.
It took approx years of universal human rights before it was acknowledged that while life was improving for men, women were not enjoying the same improvements. Thanks to second wave feminism, ‘Women’s Rights’ now exist,
The degree of sex based differences between mammals varies quite a bit based on a number of factors (eg horses, we don’t divide by sex for human equestrian sports because the animal is the physical equaliser - still though, most top horses are male! But in horses the sex differences are less pronounced so the most amazing female horses can keep up with the males. Mammals with fewer sex differences are less labour intensive to rear than humans - baby horses are born and can stand up and take shaky steps immediately but it takes an average human baby about a year to do the same)
Women are, on average, shorter, less fast and less strong than men, especially in upper body strength, and most pronounced in grip strength (have you ever heard of a woman choking a man to death with one hand via a bdsm type sex game? I haven’t)
Women shoulder all of the gestational burden and most of the child rearing burden (partly due to things like breastfeeding, partly because we are genuinely better at it (on average!) partly because we are more inclined to give a shit about our offspring (whether that’s biological or socialisation is still up for debate, I’d say it’s 6 of one, half dozen of the other),
Regardless of the nature/nature aspect, women are statistically more likely to need time away from the workplace for childcare purposes and thus women as a class need employment protections so that we are not penalised by this (society as a whole needs a new generation to be born, it’s not purely for the benefit of the child-wanting individual). Even women who deliberately choose never to have children need this protection against discrimination, because 80% of women do have children, and thus most female employees of fertile age are perceived as likely to need that time off at some point.
The reason the 1970s slogan ‘Womens’ Rights are Human Rights’ makes sense is because every single human came out of a woman and most children are raised by a woman. Solving violence against women and lifting women out of poverty benefits both sexes because male and female children benefit from not living in poverty stricken, violent homes.
Transmen will, despite modifying their bodies, still need to access the same rights and protections as other females. Transmen will still be on average, smaller and slower than men (although testosterone can make them stronger if they use it as an aid to muscle building, hence it being a banned substance in sport). Transmen will still have to deal with the unique functions of the female reproductive system, they may need abortions, have babies, require maternity leave and flexible working for childcare reasons. Even if they choose to surgically remove their reproductive function they will still encounter the uniquely female experience of a hysterectomy and the consequences of such, osteoporosis, increased risk of prolapse, of early onset dementia etc.
It makes absolutely no sense to treat male transitioners as women in every legal sense when every legal protection for women is based on having a female body.
It makes no sense to treat female transitioners as men in every legal sense when they still need the extra protections for people living in a female body.
Womens’ Rights are Transmens’ Rights are Human Rights.
It’s not transphobic to want transmen to retain legal protections based on material reality.