Contraception is a very safe and effective way to enforce a law about the number of children couples are allowed to have. Any other method would be immoral. Couples should plan their families. They should use contraceptives to stop at two children maximum. Not all couples would have two children, and so for a while world population would stop growing.
Contraception for the purpose of reducing overpopulation is not the answer to stem population growth. The answer to population growth is the self control of people reproducing. There can also be benefits given to families who have less kids instead of giving government support for people who have much larger families.
Actual, real, future people, would not want to have been prevented. An ever-growing human population allows far more people to experience life.
Each and every human life is of immense value and sacred. Human life is sacred in all its forms: old person, child, toddler, baby, fetus, natural spurting of semen - thus the natural expansion of human populations should not be hindered, and naturally-large families should be encouraged worldwide.