"Women aren't funny" is an old saw, Hitchens hardly invented it. All his arguments are the same old arguments. (And picking it apart is just too easy -- nobody with any chops does too many fart jokes, for example, they're cheap. its like working blue, it's crap.) Traditionally there have been fewer women in comedy and fewer opportunities for women in comedy -- but why should comedy be different from EVERYTHING else but teaching, nursing, and home making?
And there is fun fun fun dynamic of "pretty".
Being funny often doesn't look pretty and that turns off/ inhibts a lot of women. A good example is Lily Tomlin: I've met her in person, she is one of the most physically lovely women I have ever met. You do not get this impression from her work because comic is a higher priority to her than looking nice. Its a choice she made long ago and embraced and it has paid off for her (as opposed to many women performers who opted to play the safer pretty card). She rocks. Of course, she is a dyke.
There is also the element of casting men because they're funny (which makes them look good or at least appealing) and women because they look good (and then expecting them to be funny). Guess who ends up funnier? I think its less of a problem than it used to be. SNL stopped doing it, comedy troupes in general seem to have gotten over it.
Its all starting to fade though... the more women are in the mix the less of a novelty they become. The less of a novelty, the less requisite that "The Girl" be a looker. There are still fewer women comedy writers and its frustrating, but its continuing to shift and likely won't be so unbalanced for much longer.