My grandparents on my dad's side not only slept in separate beds, but they slept in SEPARATE ROOMS ON THE OPPOSITE SIDE OF THE HOUSE. The fact that they were both on their second marriage and that they didn't care about anything but maintaining that marriage, regardless of how unhappy it was, was most of it.
My grandparents on my mom's side were divorced in the early 60s and had both been on multiple divorces by the time I got around. My parents got divorced. My grandfather on that side didn't even intend on telling my grandmother, and the court notice came a day early and he hadn't even packed yet. Rather than making a deal about it, he just packed some clothes in a suitcase and left, and they never spoke again. This was a man who had started life as a millionaire, reduced to nothing but a suitcase and a reboot.
I don't get monogamy. I have no context for it as practical.