I like Hythe and Dymchurch, and the Romney Marsh. All very relaxing and unpretentious. I find Whitstable a bit crowded and over hyped these days, (since I lived there) while Herne Bay is unchanged and a little bit run down( since I lived there too).
Alternatively if you prefer history and culture, and plenty of restaurants, settle on Canterbury. You wont go wrong there.
All depends really what you want from your romantic weekend. Sitting on top of the cliffs at Dover, with a picnic and watching the ferries go to and fro is a favourite of mine, but maybe not everyones idea of romantic!