As one preacher paraphrased it: “Those men who think their circumcision contributes to their justification – I wish they would keep cutting.” This is not a comfortable verse to talk about and I’m not sure how much to read into it. If they emasculate themselves, they won’t be able to reproduce – maybe that’s symbolic of Paul’s wish that this false teaching wouldn’t be passed on to future generations.
But I think the main point is to show how strongly Paul feels about this teaching. He’s done this a few times throughout this letter: in chapter 1 he said “let (the false teachers) be accursed”, in chapter 3 he called the Galatians foolish for accepting this false teaching, and in chapter 4 he told them to cast out the false teachers. It becomes clear that this is not just some minor point of doctrine where they just need to adjust the balance a little bit. It makes Paul really angry that people are spreading this false teaching.