In simple terms, the "problem" is when (as part of campaigns for trans rights) people confuse the distinction between gender & sex. Gender is a social construct. It is fluid and clearly people align with whichever gender they feel most suits them. "Feminine" or "female" is the term describing the stereotypical presentation historically associated with women. Gender arises where societies attribute inflexible social roles and responsibilities on the basis of biological sex, and gives us a means to describe adherence to those social roles regardless of sex. In modern western society it arguably no longer has any real meaning, as people of any sex can love & marry any adult they like, and can dress and perform whatever job they want, regardless of their biological sex.
In some cases people who feel their gender is not aligned to the sex it is traditionally associated with, may make changes to their appearance (including reproductive organs) to create the appearance of different sexual characteristics aligned with their gender. These changes are by nature cosmetic as it is not possible to change your biological sex as it is defined by your chromosomes. An individual with XX chromosomes cannot, for example, have a functioning penis & testes with the capacity to produce sperm, ejaculate and impregnate a woman, this being the primary function of these reproductive organs. Furthermore only a woman, an individual with XX chromosomes, can menstruate, produce ova, and grow a baby in their uterus.
"Woman" is a term typically referring to people of the XX biological sex or people with intersex chromosomes, whose sexual characteristics are (from a medical/clinical perspective) closest to those of a woman.
There are aspects of human life where it is necessary to provide facilities and services specific to biological sex. For example, bathrooms with facilities for sanitary equipment relating to menstruation. Women are also hormonally very different to men, with, on average, shorter stature, lower muscle mass etc, meaning they are vulnerable to physical domination by men, regardless of gender/appearance. It is therefore socially useful to segregate certain spaces for the protection of women.
In conclusion, gender and sex are different things, and the maintenance of the term "woman" to describe the XX biological sex is essential.