I don't think it is exactly rude however more couples are choosing to live together by choice or also economic factors may delay in them "setting a date", so more tact is required today.If it is someone you know very well, a family member or close friend you will know their personality and therefore should have an idea how they would react to such a question. Common phrases i have heard in Irish circles are" about time we had another family wedding", "So . . . when's the big day?" and from the brides father "When are you going to make an honest woman out of her?"!!!!!!!!