People have children early in life. Even at that rate, the population can grow without bound. E.g. if people lived forever its clear the population would still grow.
So for a stable population in the long run couples need to be having on average around 2 kids each (assuming they are not immortal, if everyone is immortal then no kids are required for a stable population).
Of course if we take "couple" very literally then perhaps we might allow people to have as many kids as they chose provided they have each with a different partner (to form a new "couple"). This seems like rule bending though.
Without rule bending one kid per couple just couldn't work in the long run.