This is Literotica, meaning that virtually all of the stories here will include at least one sex scene. However, there are probably quite a few people who wish to submit a tale or two to Literotica, but feel that writing a sex scene is a daunting task.
To be honest, it is, even for those of us with a large number of submissions on Literotica. Sex is not a cookie-cutter activity, and writing a convincing sex scene is far from being a formulaic task. Further, those who participate in NaNoWriMo (National Novel Writing Month) and its many unofficial spin-off contests may want to add a few sex scenes to their novels if for no other reason than to add precious word count, but even then, a sex scene still needs to be somewhat convincing in order to make sense within the larger work.
Although I have been writing erotica for more than seven years, I certainly cannot say that I am perfect in writing sex scenes, but I believe I have gained some insight which may be of benefit for those who want to make their first attempt at writing erotica. Specifically, there are seven elements which I have internalized and consider as I write a sex scene.
Discovery
First, sex is an activity in which people can discover a lot about each other. Taking that into consideration, there are a number of questions to ask oneself about the characters involved in the scene:
Is this the first time the characters have had sex together? If yes, then there is almost certainly a lot of discovery taking place, at least on a subconscious level, even if all the participants are very sexually experienced. For example, does a woman in the scene prefer penetration or clitoral stimulation, or perhaps both, to achieve orgasm? Does a man have a particular erogenous zone which will instantly put him in the mood (and perhaps work as a catalyst for the scene)?
How do the characters know each other? If they just met and are simply enjoying a one-night stand, that will probably guide the sex scene in a particular direction. If they are longtime lovers, they probably know seemingly everything about each other, in which case the sex scene may really be a way for them to focus on their intimate bond. If the characters are related by blood or are of vastly different ages or are at different levels of a hierarchy, that can absolutely cast a shadow over their intimate activities, which in its own way can lead to various discoveries.
Are the characters trying something new or different? If the scene marks the first sexual experience for one of the participants, that will be the predominant factor determining the progression of the scene. If a roleplay scenario is being explored, that can put some interesting twists on things, even if the participants are longtime sexual partners. The addition of new toys or implements, such as a wider-than-average dildo or a homemade sex machine or the use of a blindfold or the testing of a whip, can potentially add a lot of variety to their sexual interactions and thus provide some spice for the sex scene itself.
To borrow a BDSM term, what are the characters' limits, both soft limits and hard limits? In other words, what do they not like to do sexually and what will they absolutely not do sexually? If a scene involved approaching or especially transgressing one or more limits, there are a number of discoveries which can be made, especially on the psychological level.
Location
Equally important is location. Any particular grouping of characters can be placed in different locations for their sexual activities, and they will likely behave differently. For example, a foursome might take place in the living room of a frat house during a major party, surrounded by dozens (or more) students cheering them on, commenting to each other, and recording the spectacle (perhaps to be uploaded to a Web site later, either for blackmail or to legitimately make some extra money). The same foursome snowed in during a blizzard may have other issues at hand, such as more intimate lighting (with the scene taking place in front of a glowing fireplace), availability of scented massage oils, or the possibility of couples watching each other more closely.
In addition to behavior, each location has its own inherent characteristics which could be exploited in a sex scene. These characteristics could be physical objects, or they may be simply how the characters behave in each location. A professor and a student having sex at the professor's second home far away from the university will almost certainly feel more "free" to truly take their time and indulge in each other, whereas the same couple having sex in the professor's small campus office may be forced to have just a quickie while strategically positioned so that no one can look in the window and see them. Objects in each location may play a role in the sex scene: hiding behind large trees along a hiking trail, sharper-than-usual blades of grass pricking the skin when having sex in a field, accidentally knocking a book off a shelf while having sex in a library... and, of course, sex on a spacecraft could be nicely complicated by a lack of gravity.
Location can also impact whether and how the "outside world" intrudes on the characters. For an incest tale in which a brother and sister are in the basement engaging in foreplay, they might hear their parents walking around upstairs in preparation for a dinner party, for example. A sex scene in a hotel might include a siren from passing emergency vehicles. A scene focusing on joining The Mile High Club could include the pilot making an announcement about upcoming turbulence.