I would like to ask for any guidelines that could help to determine when a story can be considered "erotic". Is it based on the words, the number of scenes or on how detailed they are? Shortly, if I talk about the story, from which point on should I say "It's an erotic story about [...]"?
I'd say it's erotic if:
1) the sex is a major part of the action (not necessarily the plot)
2) the text is explicit enough that if it were a movie, minors couldn't see it.
So something which has a lot of sexual contact but the "camera" always cuts away before the deed starts wouldn't be erotic. Something which describes sex in gauzy or vague terms ("they melted into one another's arms and twined their limbs together until they fell through the clouds and rain into bliss") wouldn't be erotic either.
Something which reads like a transcript of an adult film with lots of tab A into slot B is erotic.
I guess there's no answer at all to your question, unless you are writing an erotic story. If you are writing an erotic book, by definition, it's erotic since you will do it that way.
Some countries/cultures have a much easier concept on what's erotic and what is not. Maybe in USA, your book would be classified as erotic, and in France it would be quite normal.
I remember I read a wonderful fantasy book where the two main characters were gay and lovers, but it was only a fantasy book. For me, the sexual orientation of the characters didn't play such an important role to classify it as anything else than fantasy. Even so, I was quite chocked when I saw it classified as gay literature in Amazon. Like I said, it depends a lot of where and who are tracing the lines.
In any case, the answer lies in my first phrase. You can't know how people will classify your work, but you know how do you classify your work. If you write an erotic book, it's erotic.
Another thing, being erotic depends also on the weight eroticism has in the plot. You don't need to be sexually explicit -- or describe in details how the characters are having sex -- to make it erotic, you just have to make sure the eroticism in your book is higher than in a "normal" book.
Mooning writes fantasy in a great way but uses large doses of eroticism and spices the story in a way women love it -- her eroticism is clearly aimed at women -- and men also, since the fantasy plot is very good either. Even with that high doses of eroticism, her books are classified as fantasy.
Resuming, writing an erotic book depends on what you want to write, and the context of the readers.