Why are cis women's experiences standard?

Because they’re women from birth and are biologically meant to be the example of what a woman is. Trans women do everything to be like cisgender women, because it’s the only way for them to be treated like actual women and to be called ma’am. Yes, cis women’s experiences is the standard female experience since millenaries.

Should we exclude white women from women-only spaces because they don't experience the same thing women of color do? Of course not, because white women are also women, and therefore their experiences are relevant to women-only spaces.

They don’t have the same experience with race but they have the same experience with gender since their birth. What they have in common is their experience with being a woman, and I reply to your last paragraph with the same argument: a stay at home mom and the CEO both experienced misogyny and pressure that only women experience, just like there are some types of pressure men typically experience and can discuss together no matter they background.

Masculine trans women may experience it differently from feminine trans and cis women, but they still experience it.

There’s a difference between a masculine trans woman and a trans woman who didn’t transition. A masculine trans woman is seen like a « tomboy », and tomboys are women. A non transitioned trans woman is seen as a man.