The fundamental principle of feminism is that woman are morally equal to men. Logically the entire female gender consists of either people who believe themselves morally inferior to men, or are feminists.
I don't think I've ever seen feminism described in that way. The second statement does not form a valid dichotomy - even if I accepted your definition there is clearly absolutely no reason women could not see themselves as morally superior to men therefore invalidating this statement.
Meanwhile the fundamental principle of sexism (and racism, ageism, etc.) is that you are taking some empirical data (generally gathered informally) and extrapolating it to the entire gender (/race/age) group.
No. A -ism simply is a discrimination on the principle property in question. It may or may not be justified by anything - i.e. validity is irrelevent other than if you care about those things. A valid statement that is sexist is still valid even if one wishes to classify it as morally problematic.
An example of this would be mischaracterizing feminism per se, a basic principle agreed on by - for the sake of argument - all women, as an extreme viewpoint held by a small but vocal minority.
In other words: No True Scotsman has sugar in his porridge. I can only go by what people say. For a group of people who better characterise what you sum up feminists as Humanists would be a good choice.
OP expected to be called a sexist for making a fundamentally sexist remark, and I did so.
It's a fundamentally feminist-ist remark - not based on sex. By your definition above he is applying not to the entire set of women but the entire set of women who are feminists. These sets are not equal.
The real question here is why are you arguing with me, for calling him on his sexism, and not him, for being blatantly and admittedly and unapologetically sexist?
Because even if he is sexist, because even if sexism is a priori a moral evil, a clear reading of his statement is directed towards a feminist perspective, not a female one, regardless of whether or not you accept it as a valid one or a strawman.
But that's a question only you can answer ...
And with ease.
Read more carefully.
a feminist vision of "equality"
Important qualifier there.