I had this conversation with a Mormon friend of mine ~ lo and behold! ~ a 'Dear John' letter to a missionary ON A MISSION! I doubt someone would be so cold...still, it made me laugh (hope I don't go to hell for that).
(names and places have been changed)
I want you to know that you mean the world to me. I will always treasure the moments we spent together, and I know we will have many more of those moments in the future, considering that I'm marrying your sister Claire in two weeks! Yeah, isn't it crazy! We had wanted you to come to the wedding, but we decided we couldn't wait that long. We're getting married on May 6th in the Temple.
I know you waited for me on my mission and I promised to wait for you, but I know God had something better in mind for both of us. But we're thinking that we might name our first daughter after you. That would be nice, wouldn't it? I hope we can still be really good friends, otherwise Clair would feel bad.
Anyways, have a great day!