>I'm a bit mixed about this; it is clearly discriminatory to publish a job ad that says "women need not apply",

In general that is discrimination, but as with all things in law the general rule only exists for exceptions. One exception is a “Bonafide occupational qualification”...some famous cases involve men playboy, hooters, and Abercrombie and Fitch (though A&F was a case of discrimition against blacks, which A&F attempted to defend by arguing being white was a bonafide occupation qualification because it fit their overall marketing and target demographic).