After reading this sentence numerous times, I cannot think of any explanation for "open fire." Perhaps you could give us more information about this sentence.
I'm thinking of open-fire as the thing you say before starting an attack, a battle...
So may be, paraphrasing: "tough marriage, usually assosiated with fighting/battle"... Could this make sense in your context?
An open fire is a mode of domestic heating in which combustible materials are ignited in a purpose-built construction within the home. Most of the resultant heat vanishes up the chimney, but some proportion of the heat is radiated from the combustion process to warm the inhabitants. Some also warms the structure of the home.
The open fire, rather like marriage, is considered by many to be an out-moded institution.