Katnal1 controls a protein needed for sperm maturation; interfering with Katnal1 thus prevents proper sperm development. A drug that causes this intervention would be the type of contraceptive that prevents fertilization, rather than implantation. For many, including SPL, this is an important moral distinction.
Any method of reproductive control that doesn't involve taking human lives is a step forward. Let's hope this research is fruitful, so our population doesn't have to be.