A deep question in some sense, but I don't think you mean this in a philosophical way.
This is really not complicated. Honest. We are talking about sexism as it is manifested in an actual interaction in society. The definition of sexism as an intersection of gender and power is a very standard definition offered by a lot of scholars, activitists, and people who think hard about this. Your response is to say "show me the law that says that", which borders on the non sequitur.
I'm not 100% sold on intersectional theory myself and
@vhawk 's comment can be read as a surprisingly sophisticated critique of the framework but the correct response to ideas you disagree with is a) argue about why the conclusions are reached incorrectly from the premises or b) demonstrate why the premises are ill-founded. Vague invocations of "law" or "ethics"(!) don't even have the
form of a counter-argument.
My question for you:
1) Some people argue that sexism (or racism, or ableism, or whatever) is the result of a systemic/institutional power differential between two groups of people and the attitudes of the people involved are immaterial. What bearing does the law or "ethics" have on this question?