Many professional freelancers will go their entire career without ever once writing a story about sex. Oh, they may craft a racy scene or two in course of writing next Great American Novel, or they might someday write an article about latest in sexual research, but that's about as far as many writers will ever take concept.Then there are those freelancers who, for a variety of different reasons, have actually chosen to write (and publish) in sex genre. Societal notions of perversity aside, these writers are of same ilk as those who write about horses or gardening or education, it's just that their subject matter is often more controversial. Still, act of writing as a profession is same no matter what genre, although sex writing does require more of an open mind and is not, as a rule, for more prudish among us.
But basics - a bit of skill, a professional attitude, plenty of perseverance and a whole lot of research - are same as for any genre.
Sex markets offer great opportunities for writers at all different levels of experience. For fledgling writer there are many digest magazines that buy a lot of fairly well-written (but rather formulaic) stories. The pay for these pieces is, of course, lower than for other, higher-quality, markets but they do give beginner a chance to earn money while honing his or her craft.
On higher end of scale are markets that demand excellence in plot structure and characterization. These magazines usually pay fairly well and are for writers who are experienced at more advanced techniques like dialogue, subtle humor and deftly nuanced characterization.
In addition, sub-classifications of genre - letters, articles, stories and personal accounts - offer plenty of wiggle room when it comes to a writer's ability and/or experience. Letters and essays are more of a train-of-thought process, and thus much easier for beginner to write (and sell) than full-length stories or non-fiction.
This means that in sex markets, as in no other genre, a writer really can earn as she learns, starting at lower paying markets and working her way up to higher paying (and higher quality) fiction and non-fiction articles.