@funclesmuck
YANBU
I don't understand where the idea of sharing a bed ever came from. It is so hard to share a bed with someone long term. There's the coughing, the snoring, the farting, the quilt hogging, the kicking, the snoring, the jabbing in the ribs, the sniffing, the muttering, the grunting, the snoring, the differences in body heat.
Did I mention the snoring?
Everyone needs good sleep. So why the HELL do people choose to share a BED? 
Everyone should have separate rooms IMO!
Weirdly, many older people I know (born before 1950,) think it's' wrong' for a husband and wife to sleep separately. Fuck that.
My nan shared a bed with my grandpa for 50 years, until he died, and he snored and grunted and farted like a navvy. She complained all the time about him, and said his snoring was dreadful some nights, and he often disturbed her sleep. Yet, despite having 2 spare bedrooms for 30 years, she continued to sleep in the same bed as him.
WEIRD. 
Your MIL is a silly moo OP!
Old school/old fashioned, and very WRONG! Women now do what is best for them, and what is right for them and their health, not what society expects of them. I know a fuck load of women who have a separate bedroom to their husband. And they have great marriages. (And great sleep!)
It's lovely being able to go to bed and sit and read for half hour, and then snuggle down in your own bed, with the window open coz you're hot! And no snoring, grunting, or farting! (Only your own hahaha! ) 
As you can probably tell, I have a separate bedroom to my husband. Have done for for about a decade (since we were in our mid 30's!)