Writing for flashbacks is perfectly fine, as is the case in movies referenced by Lauren Ipsum's comment. Sometimes setting the scene is effective by setting a tone or expectation in the present or perhaps even just as a hook for the reader/viewer. An important point to consider might be, given the entire romance is in the past, is to make it seem as a story in and of itself, and the present, whether it comes before and after or just before the flashback, at least does not ruin, if it doesn't enhance, the flashback, as is the case with The Titanic for example, where the return to the present at the end changes the tone of that end from one of mostly sadness to a more uplifting and sentimental one.