For me, it's all about the foreplay and aftercare. As other people have noted, what really makes sex scenes pop out and fit within stories is how characters interact with each other and themselves in these scenes. And that comes out in how the foreplay leads up to the actual intercourse and how the aftercare happens in the end.
If I'm going for a slow-burn romance, the foreplay is extended. I focus on the bodily reactions of the characters. Does Johnny sigh out slowly and close his eyes when he gets kissed? Does Julie cover her mouth to hide a moan and arch her back as she's brought to the brink?
Body language, people. Gotta have that body language.
Like sure, you can describe how what goes where and stuff. But the reaction to the action helps bring out the emotions and scene to a fuller potential. Here's an example I have right here:
Kinda NSFW
"Jones..."
He looked up to Elaine with lowered, dazed eyes -- watching her flushed face as she squirmed in bed. Elaine gasped gently, gripping onto the throw pillow, and arched her back slowly with a roll. In return, he flickered and slid his fingertips against her slit, teasing her; he chuckled, leaning in to kiss her inner thigh, peppering kisses closer and closer.
She was already so wet, already so eager to dive in. But Jones wanted to tease more, wanted to draw it out longer; by the way she flinched against his lips and how she gripped his hair, she enjoyed it just as much as he did.