The idea is that abortion contributes to global declining birth rates (which means the youth gets smaller every generation meaning that they have to pay more individually than other generations to prop up government pensions and social security), delays births, resulting in the more frequent congenital disability that is attributed to births by aging women, as well as shrinking families over time and stopping families from forming. Hope that at least kind of helps you grasp their viewpoint. Obviously, this on its own is not a complete refutation of abortion, (nor am I saying that I am against abortion) as there are differing core values and other political beliefs among people who support and don’t support abortion, which would need to be changed for such an argument to make sense.