It may not be her fantasies but it may just be the fact that that is what she visioned. However, noting all of the forbidden romance within, one could say it is because of her fantasies, but we do not really know. Meyer wrote what she saw and transferred that into books and into a saga.
You see every writer writes what he/she sees, that is why the writers during the civil war wrote about misery and all, why writers in the romantic period wrote about nature, and that is why Stephanie Mayer wrote her dream because a writer writes with what he/she identifies, Stephanie Mayer wrote what she believed in, what she saw, and what she knew and was familiar with! She saw herself in this genre and she wrote about it, she vampires in that way and she wrote about it, and she saw vampires positively and they have a kind heart and they are perfect. Stephanie Mayer on the other hand was influenced by her dream a lot and as a female, females tend to be effected by their fantasies, and you can see that by the main plot in the saga which is "the forbidden love" you can see her dreams from the cover of the first book even! Stephanie Mayer just like any writer she wrote what she saw and imagined because a writer is somewhat an acceptable kind of schizophrenic and so she fantasized or “dreamed” this saga and she started from there and wrote about it, so I guess to some extent it is acceptable to say about any writer in general that they write their dreams, because any writer is affected by their life, background, culture, and dreams!