Natalism's immorality comes from the fact that it's about prioritizing non-existent beings over ones who already exist, namely women. Not fetuses, who are or are not babies depending your views on this. Entirely theoretical offspring of people who went out on a date this one time and didn't really click but by putting their own preferences over immediate procreation revealed their profound, selfish decadence.
Not just immoral, though certainly that: entirely unacceptable from a Christian point of view. People are not immortal, nor pre-existing souls who get bodies at the moment of conception. At whatever point a person becomes a person, there is now a morally-relevant being while there was no such being before. You're not denying potential future people the possibility of coming into being because there's nothing there waiting which is being meaningfully denied.
Further, hypothetical human beings have no moral weight whatsoever because their number is infinite and so they simply cannot count in any meaningful way. If one is prepared to adopt a utilitarian calculus, one could easily muster a sufficient number of them to prove any point at all, and, one presumes, muster an equal or greater number to establish the opposite point. For example: a world with the present number of people might be good to preserve civilization, etc; a world with double the population would present many undesirable problems for the people who lived in it. Bring your six billion future imaginary people, and I will bring another six, and let's see what happens.
I'm also skeptical of the idea that general future-oriented imperatives can possibly issue in any straightforward way as personal obligations: if you think it's easy to move from the general to the particular, you're wrong. As Millman notes, birthrates are a complex phenomenon: the advice one gives to some people has to be conditioned by the fact that everyone else in the world makes their decisions for whatever reasons feel sufficient to them. If the general imperative is to create enough-but-not-too-many, it has to take into account that some people will (irresponsibly!) have too many children and, therefore, someone will have to have fewer.
The point is not that any of these are unanswerable objections: the point is just that the issue is, and has to be, more complex than its made out to be.