This question applies to many genres and media. Television executives probably face this question on a daily basis and all movie directors have to consider it when it comes to certification and their target age rating.
If something I write is too rude, causes offence or outrage, then my sales may suffer. Of course if I can cause enough outrage it my give my book sales a major boost, but then I run the risk of Amazon kicking me off their site. If my novels are not rude enough my readers might get bored and may not read my next novel. I'm acutely aware that I need to land my novels somewhere in the middle.
If you are here looking for an answer to the question I posed in the title of this post, you're going to be disappointed. The only guide I have to go on is feedback from my readers. I know my writing is racier than some erotic fiction out there, especially when it comes to the psychological aspects of arousal and desire, but I know of other authors who go into far more graphic detail than I would dare. After hundreds of sales of my first novel (Remote Control), I have received a few lovely compliments and one delightful "demand" for a sequel" but not one complaint saying my sexy scenes were too soft or uncomfortably explicit.
If anyone knows how to balance what readers are expecting (which they will rarely admit to) with what you put down on the page, I would be intrigued to hear about it. I think, so far at least, I have been using gut feeling more than anything else. My aim was to allow readers to fit themselves into whatever part of the fantasy they most prefer. They can imagine they are in control, making Natalie do the things she so badly wants to do but has never been brave enough to try, or they can imagine they are Natalie, being encouraged and coerced, against her better judgement, to explore her exhibitionist tendencies and sexual fantasies.
In that sense my writing avoids the whole point-of-view issue. Readers are free to assign whichever point of view suits them. I don't know any other authors who allow that in their writing. Too much graphic detail might actually be unrealistic given the scenario I present, so the art is to find the right balance to make the reader feel that they are actually there, without cluttering the dialogue with too much detail.