But the term "Believe women" is an extreme on itself that only focusses on one part of the problem as it only envelops one gender.
And if a term focusses on a problem that is common for both genders but chooses to only focus on one side of it then it can even be considered sexist,and to make the point you suggest women do not have to be singled out for it.
Some people think that those types of movements should be driven for the people that need them most. (I don't agree with that though.)
They're not saying that men don't also have those issues, but that women tend to struggle with those kinds of things a bit more.
The only reason I would consider a movement sexist for only focussing on one side, is if people on a side tend to diminish the bigger part of the issue.
If people from a movement dedicated to men being open about being raped, started attacking women who got raped, then I'd at least start to question their intentions, I could see that movement being considered sexist.