The urge to reproduce is partly induced by the direct environment, but its main cause lies in the way living beings are programmed:
Without offspring a population will die, therefore this 'urge' is 'build-in' in our genes. The environment can change whether someone will or will not produce offspring, but for the most people it only changes the age on which they'll produce offspring. Although this too can be linked too genetical causes, for in 'poor' countries and areas in which 'civilization' is still unknown, people tend to get children much earlier than in 'civilized' countries/areas and also much more children than what is common here. It's almost like we've some internal program which becomes active when it is no longer necessary for the population to grow. That's why the growth of the world-population is caused by Third-World countries and not by Western countries.
A 'marriage-clock' would be impossible, for 'marriage' is something we Humans have invented and isn't known in every culture.