I want to know if it is possible to retain monogamous marriages and relationships if the chances of a female getting pregnant reduce dramatically which will, in turn, affect the survival of human race. 

Edit: My story is set in a dystopian world, where humans are advanced enough to have known concepts such as marriages and monogamy. Survival of the race is tough though because of the dystopic premise. My question is, would monogamy be a luxury when the human race is trying to ensure its survival? Assume resources are available aplenty. 

Edit 2: The fertility would be about fifteen days. (I initially set this as two months, but as it would be illogical to have a proportionate increase fertility window and have it impact the birth rate, I have lowered it to fifteen days) The gestation period remains the same.