They are not married. It's not like they don't or ever wanted to. Their reasoning as to why they are not married is actually ridiculous: They forgot.
That's it.
They got engaged, Mikhail proposed and everything. They both just forgot to actually get married.The only reason they realized that they never got married is due to their great grandson, Axel, asking when their anniversary is.
"..."
"I do not believe we never had marriage, yes?"
"I knew we forgot something..."