This is a controversial topic. Some people hate seeing it in fan fics, others refuse to read a story if it doesn't involve sex. In my opinion, if you are writing for an adult audience and it focuses around a romantic relationship, then sex is a factor. Sex is a healthy part of a romantic adult relationship. Ignoring or skipping over it means downplaying a major form of connection/intimacy.
The big question is: How do you portray it? I'm not going to pretend that I have all of the answers. Different readers have different tastes. That being said, I've gone through various threads, articles, and posts to try to compile a list of points that I feel are relevant.
How often is too often? In relationships, the number ebbs and flows. When it's novel and the relationship is newer, it'll be more frequent. As a couple reaches another level of intimacy, it may drop down a little. Add young kids to the equation, and well.....
So, how to convey this in a story? Well, I think a common rule of thumb is that if you do not want your story to become something out of a porno, don't write about it in every chapter. People will get bored. A loyal reader is more likely to read a story because the story line is good, than because the story is about . If that's what your story is about, then chances are, eventually at least a few readers will go When that happens, something went wrong. Novelty wears off. It is better to have no sex scene than one that doesn't further your plot. If you are going to have a sex scene, make it have a purpose. If it doesn't fit, don't write it.
Pearl. Heart. Pussy. Vag. Love. Spot. Mound. Treasure. Get the point? There are a lot of words that one could use when writing a sex scene, but that doesn't mean you need to use them all. Remember that your choice in words sends a subtle (or bold) message to your readers. Dick. Penis. Sword. Spear. Member. Rod. Dagger. Trunk. Cock. One sounds factual and direct, another sounds like one is stuck in a harlequin romance novel, another like a furry story, another, like a porno. Make sure the words you choose are consistent with the message you want to convey. This is the same for your characters' actual dialogue.
Real life sex is funny, soft, clumsy, loving, passionate, hard at times, quick and much more.
It's not always perfect and sometimes it's just like . If the chemistry is good, more often than not it is like . Yes, most authors know that people want to know what happens, but don't forget that there is more to writing such a scene than simply Action A, Action B, Action C.
Think about other details, such as the senses (taste, touch, sight, smell, sound) and where your characters are in space. It's a little hard for him to kiss her stomach if he is behind her, so don't write that. Make sure what they are doing is physically possible without major Frankenstein-ish surgery. Also, sex involves more than just the sex organs. Sure, there are the main attractions, but don't forget the side-roads. Think: hands, feet, back, stomach, etc. Think foreplay before you even think about listing exact sizes (on him or her). Scratch that, don't list exact sizes, period.
Now, if you end there you have a basic sex scene. It's okay, but it's a bit flat and dry. Internal introspection can go a long way to adding depth to your scene. Sex is more than just physical and great sex also involves the exchange of emotions.
I've just described a lot, but I want you to keep one thing in mind: a longer scene doesn't mean that it is more believable, just as more frequent sex doesn't mean that the plot is better.
But you haven't answered the question: When?
Actually, I did in the first segment, but it is worth mentioning again. Rushing to sex doesn't make the story sexier. It just makes the pairing look shallow, the attracton more superficial, (usually) makes the story less believable, and lessens the actual sexual tension that should preceed the act. Point being: give the characters some time to develop. And by "time", I mean chapterS. Going back to my opinion: a dozen, two dozen, even three dozen +, aren't too many chapters to wait if the plot and delivery are on point. That doesn't mean that you need to constantly tease your readers. That's fun at first, but at a certain point it gets manipulative and turns readers off. If there are any flaws in your delivery, they will become magnified and your number of reviewers will drop off. What waiting for the first sex scene does mean, is that you have the time to create a pairing that is so convincing and tangible that the readers can see the chemistry without you having to throw it in their faces.