No, this has nothing to do with dialogue."It would seem that it would be such a fine line to have a film with a subject matter being porn, and to have this really explicit language, and yet have characters that are so completely endearing"
If there is a fine line between A and B, then there are situations where the definition of one may slightly overlap the definition of the other. As a result, there are times when it may be hard to distinguish the two. For example:
There is a fine line between corporal punishment and child abuse.
Corporal punishment is punishment that involves striking or otherwise inflicting physical pain on someone. The Collins Cobuild English Dictionary goes so far as to use the word beating in the definition.
So, your little Johnny comes home from school one day with his legs and buttocks black and blue with bruises. Which is it?
In the case of this movie, the fine line is the fine line between a pornographic movie that is little more than a lowbrow vehicle to portray sexual activity and a movie that, while being sexual in nature, is an honest attempt at artistic expression with endearing (read well-developed) characters.
Hope this helps.