Has anyone else noticed when reading a sex scene in a book, you begin to visualise the action being described to you, only to realise halfway through you've got it completely wrong and they're in a completely different position to what you thought? I've read a fair amount of sex scenes in books over the last few years and unfortunately this happens a lot. I'm sure that despite being a little hot under the collar it's not wholly my fault for misunderstanding the action!
The fact that the reader naturally visualises a sex scene in a more detailed fashion than they might any other scene in the book can cause problems for the author. It means they must pay much more attention to ensure everything that should be obvious is indeed so, that no steps are missed out in the dance. Of course when one has sex it can be a whirlwind of emotions and clarity of thought isn't always an option, but when writing and reading about sex it should be.
The best sex scenes, in my opinion, are the ones that best allow you to visualise the action as a fly on the wall, or even visualise yourself as one of the characters. Any movements should be natural and should make sense, otherwise the reader will struggle to follow where everybody's limbs are. It's all too easy to write ambiguously, perhaps causing the reader to think the female is on top, only to then be told they have their head in the pillow. Once the reader is having to readjust their mental picture the illusion is broken and they don't feel a part of the scene any more.
The best way to overcome this problem is to go through your sex scene line by line, make sure it's obvious roughly who is controlling the action and from where. If it helps you should act it out, I'm happy to admit I did this when writing my sex scene. It can be a great help to stand up, sit down, lie down, flail your arms and legs around to see where the other character should be and what you can reach with your appendages. It's unreasonable to assume that all your characters have the pliability of a gymnast, your readers certainly won't.
If something seems a little far fetched, or too much of a stretch (literally) then consider revising it. If you want the two characters to connect in a certain way, or certain body parts to touch, think about how best to make that seem natural. Also if you have your characters swapping positions almost constantly, consider what that would actually feel like. There's a fine line between frantic, passionate love making and a routine so exhausting you never actually get to enjoy any of the positions you're being put into!
Finally, despite sex in novels often being about escapism, or about a fantasy version of love-making, do consider the difference between how sex feels and the actual act itself. If we accept that a huge part of the enjoyment comes from our own mood, a new partner, a daring situation, the sex itself doesn't need to be that different. A character succumbing to an illicit affair in a hotel room with a handsome stranger is likely to enjoy it regardless of whether that sexual partner is particularly proficient.
So when you're writing your next sex scene, put yourself in the shoes of both characters and the reader. Visualise the moves and if necessary go through the motions yourself to decide what actions seem natural. If you follow these steps you'll write a scene which is so much more realistic and thus easier to visualise and feel involved in. Your readers will definitely thank you for it.