The best response to explain why pushing the south pole of two magnets together provides a good model for the fusion of two hydrogen nuclei is:
The positive charges of the nuclei make it difficult for the nuclei to fuse, just like the repulsion of the two magnets.
This response effectively captures the concept that like charges repel each other (the positive charges of the hydrogen nuclei), making fusion difficult, similar to how pushing two like poles of magnets together results in repulsion.