It is certainly not anal sex, or any other method proposed by the pope, Malthus or Sismondi. A hint comes with the following:

Surplus population, being undoubtedly a contradiction (along with surplus production and surplus consumption) and being an inevitable result of capitalist accumulation, is at the same time an indispensable component part of the capitalist machine. The further large-scale industry develops the greater is the fluctuation in the demand for workers, depending upon whether there is a crisis or a boom in national production as a whole, or in any one branch of it. This fluctuation is a law of capitalist production, which could not exist if there were no surplus population (i.e., a population exceeding captalism’s average demand for workers) ready at any given moment to provide hands for any industry, in any factory.

In other words, the economic system in question provides the possibilities and limits for population. Therefore, if you want to change the number of people, a few condoms or the use of other orifices won’t quite do the trick; you need to change the system itself.