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First of all: it's your choice how far you want to go into detail. When two characters having sex is a plot point in a story which is not supposed to be erotic or not even romantic, a detailed de...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/14111 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
First of all: it's your choice how far you want to go into detail. When two characters having sex is a plot point in a story which is not supposed to be erotic or not even romantic, a detailed description of the deed can seem out of place, especially when you aren't really comfortable writing it. This can go so far as to just imply that sex has taken place (chapter n ends with a flirtatious dialogue between the characters, chapter n+1 starts with them waking up next to each other) or handled with a single sentence ("Then they went to his hotel room and had sex."). When you decide to go into more detail, for example because you want to use the sex scene itself to further character development or just because you think a bit of [fanservice](http://tvtropes.org/pmwiki/pmwiki.php/Main/FanService) can't hurt [TVTROPES WARNING], you should generally stay away from [purple prose](https://en.wikipedia.org/wiki/Purple_prose) and stay consistent in your overall writing style. When you usually don't use flowering methaphors, don't start using them suddenly just because you are writing about sex. Otherwise the scene will look out of place just because of the writing style. When you are writing for a readership which is old enough to read erotic scenes, you can assume that your readership knows how sex works. This means that overly detailed description of the exact interactions of various bodyparts is usually unnecessary (except when you want to write explicit porn). What's much more important, though, is to describe the characters feelings and impressions. Not _what_ are they doing, _how_ are they doing it. Do they love their partner or is it just about sex (or money, power, duty, fear, something completely different)? Are they sensual, aggressive or even violent lovers? Are they shy and ashamed or openly expressing their desires? Do they care more about satisfying their partner or more about satisfying themselves? Do they cuddle afterwards or get dressed and leave? These aspects can tell much about the personality of your characters and their relation. For further reading I can also recommend the article "[So You Want To Write A Sex Scene](http://tvtropes.org/pmwiki/pmwiki.php/SoYouWantTo/WriteASexScene)" on TVTropes.