Even before writing about the sex in a sexy story you have to set the stage, decide where this hot and heavy action is going to take place. What a lot of merry pornographers don't realize is that the where can be just as important as the what in a smutty tale. In other words, to quote a real estate maxim: Location, location ... etc.
Way too many times writers will makes their story locales more exotic than the activities of their bump-and-grinding participants: steam rooms, elevators, beaches, hot tubs, hiking trails, space stations, sports cars, airplane bathrooms, phone booths, back alleys, fitting rooms, cabs, sail boats, intensive care wards, locker rooms, under bleachers, peep show booths, movie theaters, offices, libraries, barracks, under a restaurant table, packing lots, rest stops, basements, showrooms -- get my drift?
I know I've said in the past that sexual experience doesn't really make a better smut writer, but when it comes to choosing where your characters get to their business, it pays to know quite a bit about the setting you're getting them into.
Just like making an anatomical or sexual boo-boo in a story, putting your characters into a place that anyone with a tad of experience knows isn't going to be a fantastic time but rather something that will generate more pain than pleasure is a sure sign of an erotica amateur.
Take for instance the wonderful sexual pleasure than can come from screwing around in a car. Haven't done it? Well you should because after you do you'll never write about it -- unless you're going for giggles.
Same goes for the beach. Ever get sand between your toes? Now think about that same itchy, scratchy -- very unsexy -- feeling in your pants. Not fun. Very not fun.
Beyond the mistake of making a tryst in a back alley sound exciting (it isn't, unless you're really into rotting garbage), setting the stage in a story serves many other positive purposes. For instance, the environment of a story can tell a lot about a character -- messy meaning a scattered mind, neatness meaning controlling, etc. -- or about what you're trying to say in the story: redemption, humor, fright, hope, and so forth. Not that you should lay it on so thick that it's painfully obvious, but the stage can and should be another character, an added dimension to your story.
Simply saying where something is happening is only part of the importance of setting. You have to put the reader there. Details, folks. Details! Research, not sexual this time, is very important. Pay attention to the world, note how a room or a place FEELS -- the little things that make it unique. Shadows on the floor or walls, the smells and what they mean to your characters; all kinds of sounds, the way things feel, important minutiae, or even just interesting features.
After you've stored up some of those unique features of a place, use special and evocative descriptions to really draw people in. Though quantity is good, quality is better. A few well-chosen lines can instantly set the stage: an applause of suddenly flying pigeons, the aimless babble of a crowd, rainbow reflections in slicks of oil, twirling leaves on a tree, clouds boiling into a storm ... okay, that was a bit overdone, but you hopefully get my gist.
Once again: location is not something that's only important to real estate. If you put your characters into an interesting, well-thought-out, vividly written setting, it can not only set the stage for their erotic mischief but it can also amplify the theme or add depth to the story. After all, if you don't give your writing a viable place, then a reader won't truly understand where they are -- or care about what's going on.