Whether that sexism ultimately stemmed from attitudes where women were seen as too weak to fight or not, whether patriarchy is responsible for these attitudes, is completely irrelevant to the fact that the end result is that you have sexism against both, men and women.
The types of sexism are not entirely the same, because yeah, I agree with you, there is an element of seeing women as incapable that is not present on the side of men, but that is countered by the fact that men are expected to play the protector, even at their own demise.
I mean even here:
You're basically saying: "Feminism wants to free men from sexist expectations." (a notion that I only partly agree with), but still have to phrase it in a way that completely negates the idea that men are - not can be, but are - victims of sexism under current circumstances although that is exactly what you are saying.
The way you simply can't admit to yourself that while women are victims of sexism, 'men are victims of sexism, too', and must instead reframe it to 'men are victims of the patriarchy backfiring', shows exactly why feminist theory is dangerous.