Smut basics
You as a writer have power over people’s minds. You can shoot a puppy and make people laugh, cry, puke, or get off.
But your readers aren’t psychic. There is no big glowing sign saying ‘applause’ or ‘ovation.’ You have to make that. You have to put emotions in the text and especially the subtext for the readers to pick up on.
I have read bad smut and good smut and stuff in between. The difference is in the writing. I have read boring smut. The difference between the boring an the interesting depends on the story (this part will be addressed later. Boring smut has always trumped bad smut in quantity and the reason is the subtext.
When evoking feeling form the audience, they don’t want things to be blatant. When making a movie, tricks such as lighting, lenses, music, and colors are used to alter the audience’s mood. Similar tricks can be used in writing. Use words that match the tone you want to convey and look at the connotations as well as the denotations of the words you use.
Since your main goal is to evoke a feeling, it is probably best to use some words that hold the heaviest connotations: adjectives and adverbs. Sex scenes of any kind, romantic, emotionally explosive, bittersweet, cruel, fun, or anything else require a lot of description and imagery, be it in mood, action, or analogy.
Be careful, though. Using adjectives and adverbs can make your writing more purple than passionate and can ruin not just the sex scene, but the whole damn story.
Endrael
Subtext and behavior are god in conveying love. Flower speeches are just flowery speeches, but when you have actions and emotion to back it up, then it becomes real for people. This is why Romeo & Juliet fails, or at least Hollywood portrayals of it. All the love is portrayed through flowery speeches and no real actions. You can talk about love all you like, but if you then go out and kill someone, which action are people going to pay more attention to?
Take the puppy example again. Your characters and your audience can, in fact, react completely differently to this scene.
You can in fact do two things at once and magically make the whole thing more powerful, when done right. You can manipulate the feelings of your characters and your audience simultaneously with different results.
Your subtext has to convey how your characters feel and how your audience should feel in a different way. The audiences emotions are affected by both what happened and your characters actions. Although your plot or other characters aren’t that far in the story, your audience is. Your audience is reacting to the puppy and your character’s actions about the puppy. Your characters or your plot isn’t yet reacting to your characters reactions.
This is very important to consider when writing anything, but it always pertains to sex scenes.
Edit: If you're going for titilation, a sex scene is often 'hot' due to foreplay. Foreplay can be everything from talking to mood to mannerisms. There are tiny, tiny things that can be romantic, and there are tiny, tiny things that can be very very hot.