"male" jobs (i.e, manual labor, manufacturing, etc.). So there are simply fewer jobs for men to do, especially because women are now competing with them for the jobs that are left.
And there are other reasons, like you said. Related to this is the fact that corporate workplaces have become more comfortable for women than they were 30 years ago or so, and they've correspondingly become soul-crushing places that many men want no part of.