I think I have a problem with this on two levels: constitutionally and logically. From a Constitutional point of view, there is an issue of freedom of religion (and then, by extension, if it is legal and deemed not harmful for religious purposes, it should be legal for others as well).
From a logical point of view, their argument has a logical flaw. If the infant is not capable of speaking for themselves and disagreeing with the circumcision, then therefore, an unborn child should have the same rights accorded to itself. In other words, if you can ban circumcision for humanitarian and equalitarian reasons, you must therefore ban abortion for the same reasons (unless, of course, there is a serious medical danger to the mother, in which case it should be allowed). So which way do they want it - yes to both or no to both? And even then, it may still be permitted under the Bill of Rights, but they have painted themselves into a corner, legally speaking.