Why should it?

That’s asinine, you’re treating periphery countries like they’re glorified breeding-stock for the developed world’s work-force.

Edit: To make my point more clear, the whole reason why developing nations have higher birthrates than developed ones is because they’re developing/underdeveloped. They lack access to contraceptives, and substantive access to women’s healthcare; and they also oftentimes have economies that still rely to some extent, or a large extent on non-mechanized smallholder, or subsistence agriculture. That, or they otherwise have social institutions that allow for, or require children to enter the workforce. This means that having children in those countries is often an economic boon to a family (because they can contribute to household incomes through work), and avoiding having them can be very difficult for women.

If you solve their problem of being underdeveloped, & hyper-exploited (which you should be doing if you’re a “queermunist”), then that means that they are likely also going to be in a position where they have declining birthrates because there will no longer be an object material incentive to have children, and women who don’t want to would be able to prevent it.

The idea of shoring up a declining population “through immigration” only works so long as you have an underdeveloped periphery of peoples who want to come flock to the West, or to developed nations in search of higher wages & a higher standard of living (or just avoiding Imperialist political meddling), rather than staying at home.