1) A group of five-year-olds - This is X, my special friend. We're going to get married.
2) in a Saturday morning class with whom you've worked for the past semester - This is X, my special friend. We're going to get married.
3) your grandmother, at a family dinner - I'd like you to meet X, my fiancé(e).
4) your best friend from high school, at your house - Hey, here (s)he is, the future Mr./Mrs. me!
5) the dean of your college, at a reception for a visiting scholar - May I present, X, my fiancé(e).