Why does she want to do this?

well, there could be a couple of different reasons here. one she wants to respect her husband by only allowing him to see her body (more women should have this thinking as well as men) or she baited her man and now she believes she doesn't have to dress for him. one mistake a lot of young women make is once they get married they don't think about dressing up or for their husband. Explain to her you enjoy the way she looks in the swimsuits and the dresses she wore and how proud you feel knowing she is going to be your wife and the thought of her wanting to cover her body so it is for your eyes only is heart felt explain how you feel seeing and being with her on the beach and how it makes you want her even more knowing you will be the only one from here on. good luck and best wishes