Well, as board owner, here's my take on the subject.
The line between R and X is extremely nebulous. In writing it comes down to this - is the raw sex there for it's own sake or for the story? Is the sex in the story for the titillation factor or because it illustrates the evolution of the couple's relationship?
If the story would be just fine with those scenes cut out, then chances are they are there for the titillation value, not for the story.
As to the difference between PG-13 and R - well, frankly I haven't quite figured that out. One of the other groups I write for I had a scene where a woman was raped by her husband on her wedding night - the non-consensual sex was okay for PG-13, but the description of her crying afterword with 'his seed running down her legs' was not.
BTW - the open section of this board really needs to be PG-13. It's open to everyone, including people in places where the rules against kids reading about sex are much stricter.
Besides - the curiosity to see what's in Nfic might make them want to join.
As for using 'bad' words - If it is essential to the character to swear - okay. If anyone here has seen the movie 'Hopscotch' - one of the characters is a foul-mouthed a-hole who can't say a word without swearing, and it just isn't the same with the language cleaned up. (I bet you didn't know that FBI stood for f**king ball-busting imbeciles, did you?) But what makes it great is ONLY this one character swears and everyone around him reacts as if he crawled out from under a rock. Technically, it's R rated for language - but I let my five-year-old watch it so we could both laugh at Myerson.
BTW- If you have concerns about a word being acceptable - star it like I did for 'f**k'. Which, oddly enough, makes it even more titillating.
Here is a link to the Fiction Rating System
. Print fiction has slightly different rules than film - I mean, how can you ban full frontal male nudity when is isn't a graphic or photo? The difference between PG-13 and R in this case is how rhapsodic you're waxing while describing him.