At what point did you start referring to your S.O.'s parents by first name?

Never did. In my family background, all married couples used the respective son’s or daughter’s names for parents on both sides: so—“Dad” and “Mum” or “Mother” or “Pop” or whatever. First-name address would have been both too familiar and not familiar enough.

My in-laws asked me what I wanted to call them, and I said I would address them as my husband did; so they were “Mom” and “Dad” to me for as long as they lived. I hope that when my sons marry, their wives will do the same with me.

When first introduced, his parents all said to call them by their first names. I would have gone by whatever they wanted even if they had asked to called Dad X, Mother X, etc.

My fathers-in-law (that is, my wife’s father and my wife’s stepfather) both introduced themselves to me using their first names. I took this to mean that they wanted me to call them by these names. My mother-in-law introduced herself as my wife’s mother, which meant I eventually had to ask what to call her. None of them ever wanted to be called “Mom” or “Dad” by me. They find that tradition quite strange.

My mother, on the other hand, is quite fond of that tradition and was a little sad when she found out that my wife wouldn’t be comfortable referring to her as “Mom.” My wife came up with a special name for my mother as a way of compromising. My father and stepfather wanted to be addressed by their first names, which worked out just fine given my wife’s inclinations.

Different rules for parents and in-laws.
My Mom died at 72, and it would have been much earlier if I started calling her by her first name.
She felt the title/rank/position of Mother was something to be very proud of, probably because she never knew her own.

My fathers-in-law (that is, my girlfriend has one biological father and one father that raised her (they work together in the same restaurant)) and her mother insist on me calling them by their first names.
My girlfriend does the same with my parents.
We both call our own parents Mum and Dad(‘s).
I do have to take care though, when I talk with my girlfriend and I am talking about her ‘second’ father (with her bio-father in hearing distance), that I don’t call him “your father said this and this”.
Bio father doesn’t like to hear that ‘second’ father is adressed as the father.

I never did. The older I got the older they became also and there was always that respectful distance between us.

Since the day I met her.

I refer to my own parents by their first names. Always have and never found it strange. But I know others find it very strange when they catch me referring to my mother as Judy. It never dawns on me to call other people by any sort of title beyond first introductions. I guess that seems disrespectful but it certainly isn’t how I intend it to be.

Even if I were to be introduced to Mr. President Barrack Obama… after the first intro, I’d probably refer to him as Barrack. If he invited me over for dinner, it’d be Barry from then on out. If we started playing golf together, I’d have to give him a nick name… like Boo Boo or something to get under his skin a little at the 9’s.

From Day 1.

I used first names right from the start; I could never call anyone else’s parents Mum or Dad, using first names seems just right to me, not too formal, not too familiar.

It took me a long time not to call my in-laws Mr. and Mrs. B but then I went to first names. With my DIL, I told her early on to call us by our first names and that seems very comfortable to me.

They asked me to call them Mom and Dad, but I refused so they asked me to call them by first name. I did that for a while, but I finally mellowed and agreed to call his mother Mom (his Dad passed away).

They introduced themselves with their first names, so I called them that from the day I met them. When my husband and I became engaged, however, I started calling them Mom and Dad. Still do; they’re my other parents. =0)

I never have and I never will just as my daughters lovers will be expected to use Sir and Ma’am or Mr. and Mrs. I do the same. I want them to know that I respect them and that I will do so always.

I’ve been pretty friendly with the inlaws, both batches.
First-name-basis happened in the first case cause pop-in-law was just that sort of guy and lived close by and he was an inspiration for a number of things. He’s no longer on the planet and is missed by any who knew him closely, what a character.
In the second case my wife called her mom by her first name and I followed suit.

As soon as we met, they both have everyone call them by their first name.

With my husband’s parents I always used their first names. With an ex I dated for many years in high school and college I called his parents mom and dad after several months of dating, and his grandma abuelita basicall as soon as I might her at her insistance.

The first thing my MIL said to me when we met was, “Call us Carol and Jim,” so, of course, that’s what I did. She wanted everything between us to be easy. She was a wonderful Mother in Law. Jim was a much less comfortable person, a good man, but not talkative.

As an adult, upon meeting them for the first time. As a teenager, I called them Mr. or Mrs. so-and-so, unless they asked to be called by their first names. I would never be able to call my husband’s parents mom and dad, and would really hate for anyone other than my own children to call me mom.

@augustlan I’m with that too.