Disclaimer: I know even less about sex than I do about writing. Pretty much everything I know about sex comes from the blogs of erotica writers and George R.R. Martin novels (I'm not sure anymore which I should be more ashamed of reading). That said, some things are pretty obvious:
It takes two to make it
If we lived in a less enlightened age, I'd add 'one to give and one to receive' here, but that would leave out the lesbians, and no-one wants to leave out the lesbians, right?
That you need two people to have sex (those of you lucky enough to be insisting on an 'at least' in this sentence... just shut up >_<) should be obvious. But it takes at least two people to create a story, too; one to write it and the other to read it.
The point I'm getting at is this: the little black squiggles on the paper in your book (or on the screen of your e-reader) aren't a story. The story is what happens in the reader's head when they look at those squiggles. It's a combination of the shapes and movements you describe with the concepts and reference points the reader draws from their own experience, and the analogy to sex is starting to seem pretty shaky, so let's clarify...
It's not about the physical stuff that passes between you, it's about how you make the other person feel
Which makes the book in this equation analogous to sperm, I guess? Try not to think too hard about how that would make you feel.
This is a corollary to the preceding point, really. Think about it this way: the way you transfer ideas from your head to your reader's doesn't matter. It could be via a book, or a bunch of ones and zeroes in a computer somewhere, or written on toilet paper and pushed under the partition between cubicles (the marketing technique of choice for the truly desperate indie self-pubber).
If a reader pays you for your story, they aren't paying you for paper and ink. They're paying you for an experience. They're paying you to stimulate them. Similarly, if you pay a gigolo ('rent boy' sounds so classless...), you're not paying for a sperm donation. You're paying to be stimulated.
Why do these posts always come back to prostitiution? I guess writers are just a bunch of whores...
You need the other person's cooperation for best results
I'm told that sex with someone who doesn't take an interest isn't terribly interesting. Either way, if a reader isn't willing to put the effort in, the story is going to suffer. By the same token, if you don't leave some work for them to do, you're going to crowd them out of the experience.
You can't do everything yourself. You can't be both people. You've got to trust the reader to do their part - to fill out your descriptions with images that resonate for them, to internalise the emotions you suggest, to buy into the world you create.
I've noticed, by the way, that this post is getting more and more phallocentric (yes, this is a real word and concept. I've even heard it used in university lectures). That's a problem, because more than half the writers I know are women, and stretching the metaphor to allow them to be givers (writers) is leading my imagination into painful and uncomfortable places. Let's move on.
Ultimately, it's about communication
Not necessarily verbal or linguistic communication, mind. The key to a book isn't the words or what they mean. What the characters in a story do is important to them, but it's only important to the reader in terms of the reader's responses to their actions, just as their actions are only important to you as writer because of the results you get from writing them down.
What matters is the effect you have on the other person involved; how much - if at all - you change their life. The only difference between writing and sex in this respect is that writing is normally more temporally diffuse - you write something, it has an effect on a reader later, and maybe after that the reader's response to it has effects on you (if you're lucky), whereas with sex at least some parts of the interaction are simultaneous; it flows more naturally in both directions (though possibly not in terms of literal fluids - again, I'm no expert).
Which brings me to the most important point; it doesn't have to be profound to be fun, but the best experiences all round will be the ones that are profound. The ones that penetrate right to the heart of your very being, that get right up inside you, that leave you in tears or ecstasy, that make you talk to God or feel at one with the universe (you get the idea). A good book should turn you on. Good sex should rock your world.
And one final point:
The only people who should have a say in whether you do it are you and the other person involved. Both sex and the writer-reader relationship are personal and private. They are nobody's business except the people involved, and anyone who says differently is flat out wrong. Give your stories and your love to the people you want to give them to (as long as they say it's OK, at least...).
The big difference, of course, is that children do need to be protected from sex (we can have a debate about exactly how much protection is needed, and when, where and how it should be administered, but all kids need at least some protection). No child needs to be protected from a story.