There's a contest every year for the worst sex scenes in the English language. (I suppose they limit it to the English language so as to keep the number of submissions in the thousands, rather than in the millions.) The Literary Review for Bad Sex Award
is there for a reason — to help readers and to save writers from themselves.
For readers, badly written sex scenes aren't really so bad. It's true, they can make a book you enjoy unreadable. They can make an author you have considered sexy, judging from a glimpse in a bookstore or an attractive, perhaps broody, jacket photo, suddenly seem less attractive than the guy next door with a comb-over and gaping bathrobe. (You look at the photo in horror: You like what?) It's true that when you read a scene that had you steaming your glasses at 20, re-reading it 10 years later may cause you to reevaluate the author. Re-reading it 30 years later may cause you to reevaluate the author and your early relationships. Also, badly written sex scenes can have a negative effect on your private life. Descriptions of throbbing, pulsating, magnificently heaving, and/or rigid anything tend to stir laughter, not desire. Using words that should be reserved for unusual entrees or Architectural Digest or ICUs to describe anyone's private parts (aliens excepted, of course) is not encouraging. But still, you can stop reading without hurting anyone's feelings. You can throw it into the trash. You can de-friend it, permanently.
And, if you are a young person between 11 and 15 (depending on your gender and your city), it doesn't matter how badly written it is. You can highlight it, put it under your mattress, Google the new words, and generally make some use, however misguided, of it. If you are a young man and you take your cues from the later works of Updike, Bellow, and Roth, things might not go so well. Don't say I didn't warn you. (Shakespeare will serve you better.) If you are a young woman, read with discernment. Rita Mae Brown's Rubyfruit Jungle and Judy Blume's more sophisticated works are probably more useful than so many sex scenes that are either written from the boy's point of view (She writhed with delight!) or so grimly from the girl's, you may wonder that anyone wants to have intercourse ever.
The people who really suffer from sex scenes are writers. Even bad sex scenes are hard to write. You not only have to imagine your people doing whatever they're doing (I close my eyes, even while typing, both to imagine better and to spare myself embarrassment), you have to think about doing things you may not yourself wish to do and you hesitate (wisely!) to turn to your spouse or partner and ask them to do things, whether delightfully outré or deliberately unpleasant, that the two of you have previously eschewed.
Now that you have at least pictured it and imagined your people experiencing it, you have to find words to make it beautiful OR horrifying OR hilarious (rare, but welcome, I think) and, above all, interesting. Reading poetry helps. Re-reading D. H. Lawrence does not. Listening to Tony Bennett's duets with k.d. lang may help. Listening to Boy George (unless it evokes a very special night) usually does not.
And then, you have to make it mean something and show something. Your characters have to make love, and make remarks, in ways that reveal who they are. The reader has to discover in their sexual behavior new, not always appealing, aspects of the characters. Our people have to be more than nude, they have to be naked, and with intention and meaning and heart — and no bad writing to ruin it.
Like the real thing, to be great takes practice and something more.