I'll tell you what I told a room full of policemen, FBI gents, and other assorted public safety writers last weekend:
It's all about the story, baby.
If you need to write the hottest sex scene since Last Tango In Paris, then write it. If the story requires it. If the story doesn't, then you're probably writing porn. Which is okay, but you should warn the reader in the title or on the cover somehow.
Mark Ruffalo--sexy feminist and eco-champion! He keeps his clothes on in most roles...
If your main characters are a man and a woman on a crazy adventure and they end up in hotel room after hotel room alone together, but the story doesn't need them to get sweaty and naked, then just help us readers understand why two healthy single adults aren't getting all hot, and keep moving.
Aishwarya Rai--most beautiful woman on earth. She keeps her clothes on in most roles...
By the way, if you believe as I do that every scene needs conflict, writing happy sex scenes is a rare thing.
The sex scene in If... (directed by Lindsay Anderson) was the opposite of hot for me, though Mr. McDowell felt differently.
My husband has complimented my hotter passages, but then he'd have to, wouldn't he? Each sex scene uses the pursuit of sex as an objective correlative (symbol, stand in, representation) for the conflict that must be resolved in that scene. I often end the scene before having to write about tabs and slots, but that's because the conflict is usually resolved long before everyone's clothes are off.
For some reason, my short stories are not super hot. The "Johnny Depp Kickline of Doom" story in Ellery Queen Mystery Magazine came out naturally curse-and-sex free. Oh, and my literary short story, Something Shiny. That was about a dead soldier's mother struggling with a shopping obsession. Hmm, I guess you can read my short stories without fear of hotness. Hope that doesn't put you off!
But stay away from the novels and screenplays, they always get steamed up--because desire is one of the best ways of revealing character and moving a story forward. Don't say I didn't warn you!