Good question, to which I'm sure there is a scientific answer. But using simple sarcasm, I think those that say "You get a 6 because that was unbelievable," must think that they are literary editors.

On the same theme I wrote a story that I thought was totally unbelievable and I got reply back saying "So what happened after you got to her place?'!!!

That's fair enough, because some of us are trying to write literature.

You need to write something that is believable within the framework you created. For example, if you establish the wife is super uptight, it's not going to be beilevable that she screws those two guys with no provocation. And that's something a lot of people do on this site. And reading porn is generally different than watching. Watching it, it's all visual. The visuals are what's getting you off. When you're reading a story, more has to be done with the characters. You have to craft an actual story, rather than just throwing a bunch of naked flesh on a screen.