My thoughts so far...
Real sex, if it's good, takes place in the present. In other words, it's very hard to capture because while it's happening you're not referencing it to anything else past or future, or external to what is happening right here, right now. So it's hard - if not impossible - to capture that and put it into words, whether you're trying to remember that wonderful night of passion while waiting at the bus stop, or recounting it to a friend, or putting it onto the page.
The best you can hope for is for the reader to be so caught up in the experience they get a sense of what's going on. But reading about sex is never going to be like having sex. It can't be. The most the author can hope for is to stimulate the same emotional and physical responses in the reader that they would experience if they were having sex.
At which point the author has another problem. What emotional or physical responses I personally experience may not be the same as those you experience. What I like from sex may be quite different from what you like. So if I write my real experiences of sex, you might be going mmmm or yuck or somewhere in the middle.
So, I think you can't write real sex. When I write about sex I'm deliberately vague as to what is actually going on. No body parts! No instructions! What I'm hoping to write is something that makes the reader think back to good sex they've had, how it made them feel both emotionally and physically. It's more like an impression, an outline for the reader to colour in by adding their own personal details.
Now, some people might think that's a cop out, like photographing porn through a soft focus lens. But I believe that, through having to do some of the work, the reader engages more readily and the fictional experience of sex can come closer to their own experience of real sex, without the distractions of the author's experience. So I hope my fictional impression of sex recalls real sex in the reader's mind.