From any reasonable feminist perspective, the term sexism refers to discrimination against women based on her sexual features in the context of an unequal and discriminatory society. The term can be traced back to second wave feminist discourse.
In few other discourses, the term is used in a universal way, i.e. discrimination against a person or a group of persons based on their sexual features. Apparently most of these usages have been by extremist groups.
So to hit the nail on the head always, we suggest that you do not use "sexism against <non woman genders>". Instead speak of "sexual discrimination against <person/person category>" if that corresponds to your intended meaning.
This would mean that for speaking about sexual discrimination against women and transwomen, "sexism" should be preferred. For all other gender categories including transmen, "Sexual discrimination" should be used.
Often people confuse gender discrimination with sexual discrimination and sexism.
Thus the Wiktionary lists four unclear definitions:
- The belief that people of one sex or gender are inherently superior to people of the other sex or gender.
- Unfair treatment or discrimination based on a difference of sex or gender.
- Disadvantage or unequal opportunity arising from the cultural dominance of one gender over the other.
- Promotion or expectation or assumption of people to behave in accordance with or deviate from a gender role.