What's the difference between an acceptable sex scene and erotica?

I have found that many authors have trouble drawing a line between what is expected of their genre and erotica.

The key is the answer to the question: how much sex is contributing to the story? People who sell books have to classify your work, somehow. When they look at it, they are going to want to put it into a certain section of their book store (unless you self-publish but more about that in a minute). When you include a bunch of sex for the shock value alone, you have made it difficult to classify your book. Chances are good, a traditional publisher or agent is going to ask you to get rid of some of it.

You may think if you self-publish you can avoid someone chopping apart your art-form. However, you are not thinking about the people who will be buying your book. If you say, "this is a science fiction" and then you fill it with numerous sex scenes that do not contribute to the plot, you are only going to attract those who enjoy both science fiction and erotica. You have reduced your audience. Sometimes you can combine genres and it works out okay, but erotica is one of those genres that is still very taboo. If you combine anything with erotica, it is still erotica.

One final note. If you want to publish to a Christian audience, you need to get rid of all the sex in your novel. I have actually read a book that was published trying to attract a Christian audience and it contained a hot and heavy scene. Even if this occurs between non-Christians or married people, no Christian organization will promote it. Its a dog-eat-dog world when it comes to publishing. Why cut out any of your options just to make a point?