What to tell a 6 year old?

My daughter came home from school today saying that she knew who she was going to marry. Obviously, this is quite normal and part of growing up and we often have different boyfriends. However, when we were in the car she said that she had a written a letter to her botfriend that said I love you and I am going to have with you. She has no idea what this means and I told her that it is not the sort of thing that little girls should say. She then said that someone else wrote it and she just read it. She got quite upset and could obviously see that I was shocked. I didn't go into too much detail with her as she changed the subject but I am now worried that maybe I should have said more. Does anyone have any tips on the best way to handle it without going into all the detail-I don't think she is old enough to know the full gory bits yet. Thanks.