See below. Also, as someone who is circumcised, I'm quite happy with the outcome. It looks better, is easier to clean, various sexual benefits (which I won't go into, of course), and it being a sign of the Covenant.
For those who say it would be better for men to make the decision in adulthood, I quite disagree. I'm glad it happened so far back that I can't remember it.
As you say, it's a slippery slope fallacy. Circumcision has been the practice of Jews for roughly 3000 years and of American Christians for about 150 years. If it were going to lead to other parts being cut off, it already would have. And yet, among Jews and Christians, there is no other body modification that is performed for religious reasons.