There are several aspects to this. One is control over women. Another is panic over the idea that other people with darker skin tone might overrun us with their reproductive might; either by mating like rabbits, or straight up stealing our our women. (They’re ours, right? Riight?) Also, the Book said something like "Be fruitful and multiply".

The weird thing about Great Replacement bullshit, is how it manages to mix superiority complex with inferiority complex. Like viewing oneself as less attractive than the "inferior" brother accros the street, or believing in the superiority of one’s culture, and in natural selection, and be afraid of being overrun by an "inferior" one…