You both make good points, but if you are going the route of basically saying it crosses a line when someone is harmed, then you'd have to outlaw a lot of stuff in the U.S., such as ritual circumcision in Judaism (because there is physical harm to a child that cannot give consent), and you could take that philosophical argument to the extreme, such as with piercing ears. All of these decisions would have to be made by people over the age of 18, and you could not really make an exception for kids with parents that consent because parents all over the world consent to stuff that really is harmful to children by our standards, such as ritual female circumcision in Africa, exploitation of child labor in Southeast Asia, and the suppression of women's rights in many Islamic communities.
EDIT: BTW, that explanation of U.S. policy is not something I made up. That is paraphrased directly from explanatory language on
a UN resolution where the U.S. voted in November 2010 against outlawing the expression of Nazi ideology.