I think that the ost ethical way to reduce the population of mankind is through introducing birth control, especially in the provinces and states in which the statistics shows that the majority of the females who have been pregnant is from the rural areas. By making a complex and sound plan to distribute birth control materials and proper education about birth control, it will drastically reduce the population of mankind.
There is no problem in mating, as long as there is proper and careful use of contraceptives. Some even knowing the fact, do ot go towards it. I believe the policy applied for years by China, is the most ethical way to get through the issue. Where money matters are involved people take care of things themselves with complete devotion.