Men are never discriminated against on the basis of sex.
Discrimination, a/k/a sexism, is a function of a power disparity. In the absence of a power imbalance, choosing to make Job X open to only men would be a matter of personal preference, nothing more.
In the context of male supremacy (i.e. reality) hiring only men for a job is sexism *against women.* Whereas hiring only women for a job is almost always a reflection of sexism *against women.* Yes, women.
Disbelieving a man because he is a man is not sexism. It is a reaction to the fact that sexism against women exists. Refusing to hire a man because he is a man is a reflection of the fact that sexism against women exists.
If I run a Hooter's and only hire women to run around in the little orange shorts, I am in no way discriminating against men. I am upholding sexism against women.
If I run a women's rape crisis line and only hire women to staff it, I am in no way discriminating against men. I am reacting to the conditions put upon me by male supremacy: the fact that it is overwhelmingly men who rape.
It is power, and power alone, that makes discrimination morally wrong.