The Catechism forbids mutilation, so why is non-therapeutic circumcision allowed?
The short answer
Mutilation alters the body for non-therapeutic reasons in ways that impair its functioning. God himself established circumcision as the sign of the Old Covenant, so it cannot be intrinsically immoral mutilation. Parents may still decline it in conscience, without disparaging those who choose it.
Mutilation means altering the body, for non-therapeutic reasons, in ways that interfere with its ability to function.
God established circumcision as the sign of the Old Covenant (Genesis 17:10-14), and his people practiced it in obedience for thousands of years until baptism superseded it (Colossians 2:11-14). We must assume God would not institute a rite that amounts to deliberate mutilation and so is intrinsically immoral.
Parents who object to non-therapeutic circumcision may refuse it for their sons as a matter of conscience. But they should avoid arguing in ways that cast aspersions on other parents' legitimate choice to circumcise.