Oh, I forgot about atmosphere. When writing make sure you get the feelings of both participants (or all them in case you're doing ménage à trois ). Sometimes though the sex can be angry , boring, sad (sentimental) or just emotionless . Depends on what's going on. I'll use my characters for an example. Abigail, my main character has had sex with people he doesn't love so the sex is not mutual because he's not with the person he loves (Azazel). He talks about guilt and wishing he wasn't even there. So if in you story one of the characters isn't feeling it (or both), its good to show that in some way. Rape is another issue--I wouldn't even call that.....well yeah it is, but usually that's violent however it can be used for good........... . I'm telling the truth.
Anyways remember the plot as well. If the characters are in love, then the emotion aka atmosphere needs to show that. If its angry sex then show that. Sex is (in my opinion--in literature) a really good way to show just how strong that characters feel for each other. Noble actions and kind words are great, amazing. When you chose to write [about] it (sex) can be really amazing. So there must be emotion, balance and you must take in consideration just what's going on in you characters' minds. Hope this helps