Yes, it is sinful in that it is a violation of the conscience of Jews, Ethiopian Orthodox and Hebrew Catholics who also require it. Not to mention the precendent it sets. Perhaps someone will some day feel the practice of baptizing infants is violating the rights of the child who should get to decide for himself? It is not for you or anyone else to decide that an immemorial religious practice is barbaric especially one God commanded be done. Instead of wasting time trying to curtail parent's rights, put that effort into the pro-life movement.
While I'm not advocating a complete ban on circumcisions, I don't think your reasoning that it's an "immemorial religious practice" stands. It assumes falsehood has intrinsic rights for one thing. But perhaps more illustrative, is that this kind of reasoning could be used for almost anything. For example, there have been many cults (including that of my Incan ancestors) which involved human sacrifice. In fact, human sacrifice is probably almost as old as "sacrifice" in general, being present in almost every civilization that I've read about at some point in time.
Personally, I think greater harm (relative to the harm of circumcizing a child) would be done making it absolutely illegal - Jews and Muslims should probably be exempted, or the other groups you mentioned. However the idea of circumcizing male children in general (which is a uniquely American phenomenon, which never caught on quite as much up here in Canada - and it never caught on anywhere else either, save in the Islamic world) is another matter entirely, and I would support making it illegal on the following grounds.
1) The hygiene benefits are dubious, and highly contested in the medical profession. Also, this was not the stated reason for this practice amongst the Hebrews anyway - Genesis 17:11 clearly states that circumcision was given to Abraham as a sign
of the covenant He was making with Abraham, a promise regarding his descendents, and their descendents - IOW, a promise intimately linked to the generative organ of the male. Obviously this is no longer relevent, and cirtainly not for the Christian, since the "seed of Abraham" has come and the basis of man's covenant with God now is one made in His
2) Any surgery which is not necessary, should not be performed. Circumcision is not necessary, and it was not "medical necessity" which caused this practice to appear amongst the Semites to begin with.
3) Circumcision is an extremely painful affair. The idea that it's less painful to an infant is not demonstrable - it's only because they cannot as eloquently vocalize their discontent, that we believe it's less painful. Having seen the proceedure done before and the state of the child in the period afterward, I'm hardly convinced that this lack of verbalized suffering proves anything but their lack of vocabulary.
As an aside, I find the "Hebrew Catholic" and "Ethiopian" practice to be dubious, theologically speaking. There is
no Christian basis for this practice. It is true that early on, there was some confusion on this topic as the ramifications of Calvary set in and had to be thought out - so obviously there were those in good faith who continued many such practices without it emplying a repudiation of the Cross. However, to insist on these practices for anyone
is untenable - this is precisely why certain Hebrew groups remained self segregated and became schisms, while others simply continued onward as one with
their other Christian brethren, no longer making such distinctions as "Jew" and "Gentile", since their ultimate identity and their basis for a relationship with God as sons, is established in the "covenant in My Blood". The Hebrew Catholic groups you mention, in such extravagances as insisting on circumcision, seem to be relatively modern. As for the Ethiopians, I honestly don't know enough about them, but would assume it's just a carry over from the Falashas that was just allowed to continue "as is", however theologically groundless. The only good thing is that I'm quite sure no Ethiopian would regard another uncircumcized Christian as being "un-Christian" or "less Christian" for this.