Yes there is a line, and it's easy to answer. We draw the line at consent. Any media that has non-con sexual interactions should be clearly identified as such. Consent is where we draw the line
Edit: this is also my response to the question below about necro. The corpse can't consent, and treating a corpse like an empty husk stripped of its humanity is desecration and is obviously not okay
Wrong again. It’s not about consent.
The whole porn industry is build around consenting adults engaging in all kinds of activities, from vanilla to super extreme.
It doesn’t mean this content should be depicted in media products just because they’re rated M. A hardcore porn scene depicting consenting adults would shock the audience of an Avatar movie, for example. And would in no way improve the movie.
Consent in fiction occurs between the author and the reader.