Not a thing. If you judge someone based entirely on their sex; it's just plain old sexism.
Sometimes used to suggest that sexism can only go one way (say, males can only be sexist against females) and that if a male experiences sexism it's
not really sexism; it's reverse sexism. Or the opposite of sexism.
This is designed to downplay or justify the sexism the male is experiencing.
As a term, reverse sexism is as
realistic as reverse theft. A Klan member may suggest that if a
white person steals from a black person it's reverse theft.