Let me explain briefly. Facebook allows advertisers to pay a rate per advertisement displayed. Why shouldn't Facebook 'let' their paying customers show the ads to those they are interested in. AKA why would they pay Facebook to show their ad to people they are not interested in?
That being said - I've followed most of the discussion...and there are a lot of strong points. I just think that the title shouldn't that Facebook LET them advertise in a certain way - it should be the company that is advertising the positions only to men - although they certainly could be targeting men through that channel - and targeting women through another channel - with differing types of ads all together.