What if people cut their children's arms off for religious reasons? I guess you would agree that this could not be tolerated by a free society, so why should circumcision be any different?

Bodily integrity is not an issue of family life, but a basic human right. If you want to live in a free society you must create some instance that protects negative freedoms like the freedom from harm, and that has the power to sanction violations of these freedoms, or otherwise your free society will be worthless.