Word-for-word, the sex scene is one of the most important in your novel and few genres don't, at some point, require you to address the idea of sex. Of course, you can simply choose to close the door on the reader, leaving your characters to their privacy, but don't be too quick to make that choice.
Why you should do it?
There are plenty of reasons to gloss over the sex act, such as the age or mores of your target reader, but don't use these as an excuse. If you've already put your reader through foul language, gore, and violence then they can probably handle a little sex. In fact, after all of that, they're probably looking forward to it.
Sex is the reward for every obstacle your heroes have overcome and every danger they've survived. And since your reader has been with them every step of the way, they just might hold it against you if you don't invite them to the afterparty,.
When should you do it?
Sex between the two main characters can bond them together, either at the start of the adventure or just before the climax, or it can be the spoils of a grand adventure. Of course, it can also happen anywhere in between, and sometimes it's the act of sex itself that gets the ball rolling.
Bear in mind, however, that once your two main characters have had sex, your reader will never look at them the same way. You'll never be able to build (or recover) the sexual tension between them, even if you allow years to pass. In fiction, as in real life, familiarity dampens intrigue. So if you want a sex scene in the early parts of your book, you'd be wise to keep at least one of your main characters out of it. Best to save yourself until you're sure the reader is committed.
So how do you do it?
The build up to a sex scene-the foreplay if you will-might go on for pages, even your entire novel, but the description of the act itself is usually quite terse. Within this small passage, it is important to choose your words carefully, eliminating all but the most evocative descriptions and not overwhelming your reader with detail. Kissing a trembling stomach or the feel of a rough palm across sensitive skin is far more effective than a blow-by-blow of what comes after.
The golden rule of sex, though, is to tell the truth. This doesn't mean you'll need tons of personal experience, but it's important that you start with something you've either seen or done. If you don't, what you describe is going to feel impersonal, and therefore unsatisfying.
Writing an honest sex scene is a tough job, and easily as intimidating as inviting someone into your bedroom. But, as a writer, it's your job to bring people into the world you've created. And the best way to do that is to take something from your life and use it to touch upon something in your reader's.
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