There are many factors contributing to the growth rate of the population, the first reason being the increase in lifespan.
In regions with better medical environments, the average lifespan has already reached 160 years. Note that this is the average lifespan; some individuals, who take exceptionally good care of themselves, even live to be 238 years old!
Under such circumstances of high lifespan, who would want to stay with one partner for life, in sickness and in health?
The entire cultural environment of our civilization is slowly changing due to the increase in lifespan.
Humans are creatures that crave novelty and tire of the old. The former "lifelong companionship" meant fifty to sixty years, but now it means one hundred and fifty to sixty years!
Humans will maintain their youth and strength for an extended period, and the vast majority would rather have their fill of fun before settling down to establish a family and raise children...