There are so many great adult themed fan fics out there regarding Edward and Bella. I don't know if I am allowed to post links here to other websites so I won't but PM me if you want the links to some of the stories and communities where you can read these. Also, some of us "ship" Rob and Kristen who plays Edward and Bella and there are some really great adult fics about them as well.
hey guys - this may have been posted already - but I thought I'd point you in the direction of www.twilighted.net. They have some amazing stories there, and you can filter them so that you only get the rated M ones or whatever. Also try fanfiction.(com?), and search under books or movies for Twilight. I think you'll love it, from what I'm reading here!