When you're a kid, nobody has names besides other kids. Adults only have names if they're famous and on TV and even then, it's questionable.
Teachers are Ms., Mrs., or Mr. Never first names. On the off chance you see a few teachers chit chatting in the lounge or after class and you hear one use the other's first name, you're indignant. Stacy? Who the hell is STACY? No, that is Ms. Borris, thank you. Didn't you get the memo?
Parents and family members, except other kids, have a similar name problem. The one exception is aunts and uncles, in which case their first names are Aunt and Uncle and their last names are Aunt Judy, Uncle Steve Steve, Aunt Sheila, Uncle Mark, Aunt Jenny, and Uncle Tom. Everyone else's names are Mom, Dad, Grandma, or Grandpa. Sometimes Grandpa's name is also Dad, which makes your childhood mind wonder what the fuck is going on with your family, but you're young, so you don't judge.
The first time you hear anyone address your parents by first name, it both confuses and angers you. Perhaps, they've dragged you to a grownup party and as they and their friends are on the way to deposit you into the "kids room" which is likely a musty basement or guest bedroom that hasn't been used in years...somebody died there in '03, and they haven't been in there except to put up a balloon so you don't feel like your parents were just too cheap and lazy to hire your favorite babysitter.
So, they toss you in the child depository, and on the way, you hear another adult, whose name is Mr. Garcia, refer to your father, "Hey, John, the kids are all in that room downstairs."
You rack your brain for this mysterious "John" character, thinking it's another child, only to realize Mr. Garcia is looking directly at your father. "He's not a kid, you asshole," you think for a minute, until you hear him do it again.
You put two and two together and figure out that the person you've known as Dad all these years is being referred to by this strange pseudonym. You get angry and correct everyone by saying, "Excuse me, um, there is no John here, you assholes. His name is 'Dad,' okay? Get it right!"
Alternatively, you may realize that Dad's real name is John. You've been betrayed. He's been lying to you all these years. The nerve of that prick to tell you his name was Dad. You, your mom, and your sister were the only people who ever called him Dad. Everyone else is referring to some asshole named John. What kind of double life is this man leading?
I wonder what other lies he's kept concealed. Maybe Grandpa's name really is Dad and this guy just showed up from nowhere. Maybe he had some dirt on the family and if Grandpa didn't let him pretend to be Dad, we'd all be dead because we'd have found out about the secret Dad Illuminati.
Over time, your sense if indignation fades, as you allow yourself to be brainwashed that this mysterious man, be he Dad or John or some other asshole, is here to stay and you can do little about it....
Until you're a teenager. That's why as soon as kids become teenagers, they hate their parents. They have flashbacks to that first time they caught their parents lying about their names and it's time to exact revenge on those liars....Well, that's what you get for lyin' to your kids!