While a man who wants to be a nursery nurse is being discriminated against, the root of the sexism is still in the fear/hatred/dislike/devaluing of women and "womanly" things.
In nearly all cases it's "women's stuff" that is valued less or considered to be worse, and "man's stuff" that is valued more highly.
Women shouldn't be lawyers and doctors and politicians because their little brains can't cope with it and they should be at home making dinner.
Men shouldn't be nurses or look after small children because that's women's work and is therefore crap - there must be something wrong with a man if he wants to do that.
Little girls who want to do boyish things are tomboys - it's understandable that they would want to do boy things because boy things are great, but they are reaching too high.
Little boys who want to do girl things as sissies, they must be prevented from doing rubbish girly stuff because it'll make them weak or turn them gay.
It's not that "men are not good enough to do women's work". It's that "any man who wants to do women's work must have something wrong with him".