There are the usual considerations such as how explicit do you want/need them to be? What kind of mood are you going for (beautiful, sensual, raunchy, porn), and should the scene be short, maybe even a scene in passing, or long and drawn out? These are all important questions.
For me, sex scenes are very special because they provide a unique opportunity to show character development, relationship arc, and story arc. And I believe each sex scene should be treated as its own little story with a beginning, middle, end, and conclusion. If you're going to write one it really should to count.
Character development - Everyone makes love/fucks differently. A sex scene can show a character tag, it can show what a character truly wants (and not just to get off either). It can show that someone really wants affection or is craving love or has a need to dominate or that they're angry or that they're in love (maybe not with the person they're with), what they like and are turned on/off by and on and on. A sex scene can show someone surrendering or resisting someone or something and how that character feels about it. It can show the baggage from past relationships and it can show that a character is terribly lonely.
Relationship arc - This is a no brainer. Does the relationship change at all during or after the sex scene? How? And how do the characters relate to each other? Or maybe they part anonymously, never to meet again. This says something about relationship too and about the character.
Story arc - A sex scene can suddenly raise the stakes by making someone care more or not. It can cause conflict with the character or other people. It can make things matter all of a sudden.
All of these things are true whether you're writing about a sweet honeymoon scene or a massive impersonal orgy. Both scenes are fantastic opportunities for storytelling.
As for me, a sex scene is always damn intimidating to write, and it can take me a little while to work up my nerve. I've been using the following process:
- Consider what your goal is for the scene. Are you trying to show the relationship is changing? How can you do that? By making it sweeter or rougher or something else?
- Try to keep it fresh and special. You can vary the location, the time, the situation, etc. Setting will play a huge part in the scene whether your characters are in bed with a fire going in the fireplace, in the shower, outside, or in a unused room at a huge party.
- Vary how they do it. Different positions, different types of sex, experimentation are all valid ways to show character preferences and changes.
- Once I write the initial scene, it's usually okay but not great. I make it way better by adding more and more layers of details. I'll add explicit detail, add more "love and affection" type details, if appropriate while keeping in mind character, relationship, and story changes. I work on it for a while then let it rest then do some more work. My sex scenes often go through 10 or more drafts.
- Mine tend to be explicit but it's not necessary or preferable depending on your story. If you're not going for explicit but you want to write a longer scene then I would suggest focusing on "love and affection" and sensual details. Kissing, touching, hugging, and other kinds of pleasuring. Sound, scent, touch are all very important details.
- Speaking of kissing, I have to say I still don't feel comfortable with my kissing scenes. They're not bad but I want them to be off the charts. Practice makes perfect.
This is just me but I don't like scenes which are very obviously a description of the author's sexual preferences and body types. Such scenes take me right out of the story. I don't want to hear that the author likes blondes with big boobs. I want to see what's going on with the character, how the character feels about his/her partner, and what changes are occurring. Those are the things I find interesting.
Lastly, sex scenes are a fantastic place to inject humor, if appropriate to your story. Humor can endear a character to the reader and provide additional bonding for the characters.