If you say “I’m a woman because I dress and act feminine” you are saying “A woman is defined by being feminine. A women must be feminine”. Which is the exact same thing conservatives and misogynists have been saying since the dawn of (patriarchal) time.
“A woman is an adult human female” is the least offensive, the least sexist way to describe a women. It doesn’t tell her what she has to wear, how to behave, how to cut her hair. It just states the material reality of her existence.
“A woman is everyone who acts like a stereotype that’s about being submissive and caring and wears dresses and makeup” is the most offensive and sexist way to describe a woman. Because it doesn’t allow for a woman to just BE. By this definition a woman has to perform to be a woman.