Well, since I’ve never been married, I’ll talk about my parents. 
My mom’s father passed away when she was 23, a couple of years after my parents were married (and before I came along). My other Grandfather said to her “I know I can never fill the void left, but from now on, I will be your father.” So for the rest of his life, my mom called him “pop”. Both of my parents referred to each other’s mothers by their given names, though.
Oooh I kind of love the idea of calling them “Mil” and “Fil”. That threads the needle nicely.
I call my inlaws by their first names. But then so does my husband. And he calls my parents by first names.
My parents called their parents-in-law “Mom” and “Dad”. But my wife and I always used first names for each others’ parents.
My ex-wife never really got comfortable with calling my parents Mom and Dad either, despite that being their preference.
I simply asked my father-in-law on my wedding day, “So, what should I call you two now?” He said, “Phil and Jan, just like you always have.” So it was easy for me.
I’m not married, but parents have always been called by their first name. Unless they don’t tell me their first name when introduced, in which case I assume they don’t have a name. Then I don’t address them as anything.
Have you expressed to them that you would prefer to call them something other than Mom & Dad? If so, what was their response? If not, why not?
I call my in-laws by their first names, but my sister in law (my wife’s brother’s wife) calls them Mom & Dad. I don’t think they really care either way.
My own parents are Ma and Dad; my in-laws are Mom and Pop.
My kids, from the time they were little, always referred to their grandparents as “Grandma and Grandpa from Pittsburgh” and “Grandma and Grandpa from Baltimore.” They (the kids) are now teenagers, and I still have yet to hear them refer to their grandparents as Grandma or Grandpa with their first names.
FIL doesn’t seem to care either way, so I call him by his first name, but I toss my MIL a “mom” bone every now and then (does that sound dirty? I think it does) to make her happy. I don’t do it often, but she did cry the first time I called her “mom” on my wedding day.
No way–the first in-laws were called SOBs and my current one is called “Miss ‘First name’” as is the custom in my culture.
Hell, I don’t even call my parents mom and dad.
My sister-in-law had the crappiest home life imaginable and adored my parents, so I had to grit my teeth and remain silent when she called them Mom and Dad. It still bugged me for a while, though.
I don’t think I know personally anybody who calls his in-laws by the same terms as the blood/adoption relations. The closest would be referring to someone by terms which specify a relationship different from that the speaker has: for example, calling my mother by yelling “Yaya Maiteeeeeee!” means we need to speak with the grandmother of the kidlets’, or saying “Uncle Jaime” when adressing said uncle’s Nephew. Both can be done by either side of the family and they’re basically a way to make sure that all necessary information is conveyed clearly (“I’m talking about your Uncle Jaime, not your friend-from-school Jaime or your friend-from-the-pool Jaime or our neighbor Jaime”).
I call my FIL “dad” and MIL grandmother respectively,
except in Canotnese and Mandarin.
So dad becomes “lao dao”. Perfect solution - I don’t feel like I am “mis-using” dad ( I couldn’t call him DAD), and he feels like he is getting the respect etc etc and feels good about it.
I call my in-laws by their first names, they seem fine with that.
My wife’s stepdad, though…every time I see him, he makes a big point out of saying, “Hi, son!” It’s gotten kinda irritating; I’m not his son, I have parents of my own.