Hi missjay well there’s a couple of reasons.
When we change the definition of woman from a sex based definition to a gender based definition it means we have to a) accept that gender is a real tangible thing and b) accept male bodied people under the legal definition of woman.
Feminists believe that gender is a social construct designed by the patriarchy to keep people in their place by saying it’s a real thing we are saying that women are born to do all the caregiving and wear make up etc that it’s ingrained inside us. Which is something that women have been fighting against for centuries.
When we change the definition of women to include male bodied people we lose all of our sex based rights. How could you sue your employer for example for sexual discrimination because they only hire men as they won’t get pregnant if legally men can get pregnant too.
Women are not oppressed because of their ‘gender identity’ gender identity IS the oppression, we are disadvantaged because of our sex. Being the class of people who are the breeders and feeders of the species is why we need specific laws that protect us from being sidelined by society. Maternity pay for example.
Womanhood is a biological reality, we live it with our bodies. It’s not an idea in a mans head.