When writing fan fiction, many authors find the line between “Professor” rated fics and “NC-17” rated fics to be a bit blurred. This is natural, as it’s a very fine distinction which requires thought and poise to understand.
"Professor" fictions are inappropriate for readers under age seventeen. Often, "Professor" fictions contain more profanity and violence (usually in the mild to moderate range). "Professor" fictions may contain sexual innuendo, but may not be graphic in nature.
Although profanity is allowed in "Professor" fictions, stories will be rejected if f-bombs (and s-bombs, and other vulgarities) are dropped constantly throughout the story.
NC-17 fictions will not be posted on MuggleNet Fan Fiction. NC-17 fictions characteristically include extremely graphic violence, large amounts of profanity, and graphic sexuality.
NC-17 is anything that goes into enough detail that it could be classified as a porn. You can say "they fell into bed and made love," but we don't need to read anything about him sticking anything into her. We do allow graphic imagery, so while there may be a strong implication of certain acts, there can not be minute details.
If you're still confused, think about an "R" rated movie that you have seen. While we might know that two people are having sex, no penetration is ever shown.
If you need an example, this is an excerpt that uses graphic imagery, but does not go into detail and therefore is acceptable under the Professor rating.
"The taste of blood lingered in her mouth. The pain between her legs nearly unbearable. The bruises on her arms and legs concealed by magic hurt every time he touched her. Painful whimpers echoed through her bedroom, the sounds stopping at the door and going no further due to spells that silenced her voice so no one else but the two of them could hear.
His painful grip on her hips, his cries of pleasure mingled with her screams of pain."*
This is acceptable because, while one can tell exactly what's going on from the description, it only implies the rape and doesn't detail it; if, however, the story described a penis and penetration with detail and ample description, it would be NC-17 and unacceptable.
Violence also has a limit. While "Professors" allows mild to moderate violence, once it becomes extreme it is "NC-17" and will not be accepted at MuggleNet. You can say that Harry was killed or beaten, but you can't say that a Death Eater used a large rock to bash Harry's head in or that Harry's brain was oozing out of his nose in chunks of blood and tissue. You can say that Draco was stabbed, but you can't say that Draco was cut open and his entrails were spread about in a decorative manner. Usually, a general description is enough.
Finally, there is a limit on bodily fluids. Unless it's saliva or bile with vomit and necessary to your story, do not include bodily fluids. Usually, it's only included for imagery reasons, but in reality it is rarely crucial that those details are included.
When in doubt, cut or revise the line. Find a trusted beta to help decide how to reword your descriptions, or contact a moderator with the excerpt in question. Do not be shy to ask. We’re here to help, and that includes assisting with possible NC-17 content.
*Excerpt taken from "Delightful Pain" by Queen of Serpents.