Count the number of women in the Producers section, and you'll see that television is still a very male-dominated industry. The Writer's Guild of America (West) claims that only 27 percent of film writers and 19 percent of television writers it represents are female.
Naturally, as a consequence, the male voice is more greatly represented in media than the female voice, and the audience respectively will be assumed primarily male. Main character ensembles will typically be composed of primarily males, with a Token Girl thrown in for good measure. Due to people usually understanding their own sex better than the other, said girl tends to be based on relatively stereotypical notions. Thus, The Chick is a prevalent character type. Being unable to develop characters of a different sex as well as characters whose sex matches the writers', many times the female characters end up being either Flat Damsel in Distress types, or Tomboys. This also explains the presence of characterisation tropes such as Lineage Comes from the Father or Never a Self-Made Woman.
A major effect of having male writers in greater proportion than female writers is that Fanservice tends to be skewed towards the male audience. The female character(s) are more likely to appear in Stripperiffic gear and have large breasts (even when it sharply bends Willing Suspension of Disbelief) than the male characters are to receive a Shirtless Scene. Many times, the Token Girl will be used primarily as an intentional Ms. Fanservice, while Mr. Fanservice tends to be less blatant. When a man has the power to get some of the most beautiful women on the planet to pole dance, he generally tends to use it.
Of course, for skilled writers, their sex will not affect their ability to write a cast with well-developed female characters as well as male characters, or appeal to a female demographic. They will write their female characters as actual people rather than including them because they have to or to make an Anvilicious point, and while Fanservice may not necessarily be absent, it tends to be more realistic and appealing to both sexes.
Outside of fiction, 'recognizable examples' also tend to be written with (heterosexual) men in mind. (You know, like that time that hot cheerleader didn't want to go to the prom with you.) This is especially common on geeky websites, though there are inversions.
Some women writers go so far as to adopt a masculine Pen Name— a Moustache de Plume— in the belief (or their publishers' belief) that this will make them more marketable, especially in traditionally "male-oriented" genres. It's hard to say for sure whether that fact proves or disproves this trope; either way, there's probably an element of Self-Fulfilling Prophecy involved. (And on the other hand, to be fair, male writers have been known to take female pen names when writing for the romance market, for instance.)
Very much inverted when one gets into fanfictions - Most Fanfic Writers Are Girls. Thus the number of shipping stories regardless of the genre of the original material.
One of the major causes of Men Are Generic, Women Are Special. Caused by Write What You Know. See also Most Gamers Are Male and Girl-Show Ghetto. For another type of disproportionate representation in media, see You Have to Have Jews. Compare Most Writers Are Writers.