I've found with Stephanie Meyer's writing that she likes to give everyone a happy ending. This was a hard one to find a possible way to make a happy ending, and even though its a bit of a stretch, she managed to make it work. The critical side of me is not at all crazy about the last books in the series, but she gave people what they wanted. Bella and Edward could be together forever, and Bella didn't have to give up the human experience of raising a child. And not even Jacob was left behind in the dust -- he ends up with a soulmate. So I think that's why she chose to write the final book that way, to create a happy ending.
As for the honeymoon scene, people have argued that it's not appropriate for young readers because of the sexual activity. However, I can think of many books that are much worse in terms of explicit content. Twilight is very chaste in comparison, and Bella and Edward were married. I don't know if it is appropriate for some of the very young readers of Twilight (I've seen third graders reading these books) but for most of the teen (and adult) audience out there, its perfectly appropriate.