The question of what you call your FI's parents brought about another question in my mind - What do you call your parents?
I call my parents Mom and Dad - always have, always will.
My cousin on the other hand calls her parents by their first names, ie. Jane and John. She started when she was teenager and still continues to this day (late-20s). I thought it was teenage rebellion when she was younger, now it just makes me see red when I hear her refer to them by their names or call them their names to their face (they're very lovely, caring people). But I was raised where you respect your elders, you call your neighbors Mr. & Mrs., your Aunt's and Uncles are Aunt Julie and Uncle George not Julie and George. Am I outdated?