[tsimionescu]

The words woman/women refer explicitly to a group where every single member is a woman (female).

The words man/men, at least historically, can refer either to a group of unknown composition OR to a group where every member is a man (male). Basically it is a homonym with two separate meanings, just like mouse (animal or input device) or chair (object or leader of a committee).

Point being, policeman being ambiguous doesn't mean that we would expect policewoman to be considered ambiguous as well, since woman is not a homonym.