Some people are easily sexually aroused by pictures, some by words. Stereotypes say that men usually prefer pictures, and women prefer words. Also, if the product is too obviously designed for the purpose of sexual arousal, that is considered low status.

So, the trick is to create a book sexually arousing enough that it will increase sales, but not too much so that it would reduce the status of customers; we need some plausible deniability that the customers are buying a piece of art. Twilight plays it safe, Fifty Shades of Grey tries to push it as far as possible.

Porn is not limited to just images, of course. There is a lot of porn in the form of text.

The location of the boundary between soft and hard porn is an interesting question which probably does not have a single "correct" answer. I tend to think of hard porn as being interested exclusively in clinical detail and specific particulars while having little pretensions to being an art form. Soft porn is less single-minded and, um, less hard-edged.

I don't think that "just naked" vs "having sex" is a deciding factor.