It’s easy. Fundamentally they believe the world is overpopulated, and just like any other animal of which there are so obviously too many, it’s perfectly reasonable and moral to support methods that benefit the population.
We can’t make an impact without demonstrating why human beings are more than mere animals and, sadly, Christianity lost ground on that eons ago when we failed to maintain a strong presence in the field of science. Now that we’re being forced to return to it we find, general, that our grounding in our own faith isn’t solid enough to deal with the sheer propensity of god-less worldviews. The solution is we must face our weaknesses and begin to learn again. People’s lives depend on it.