Jews have abandoned the practice as archaic (just like women being secluded as 'unclean' on their periods). Another is that though it is a Muslim tradition, it is not only not in the Qur'an, but it violates the exact phrase "We have indeed created man in the 'best of moulds'." (Qur'an 95:4) and "The One Who has 'perfected everything' He has created and began the creation of human beings from clay" (Qur'an 32:7)
Still, those two groups aside, it is Christians where the puzzlement ultimately lies, as it circumcision is expressly spoken against in the New Testament and the Bible couldn't be more clear:
"Behold, I, Paul, tell you that if you be circumcised, Christ will be of no advantage to you." – Gal 5:2
"And even those who advocate circumcision don’t really keep the whole law. They only want you to be circumcised so they can brag about it and claim you as their disciples." – Gal 6:13
"For there are many who rebel against right teaching; they engage in useless talk and deceive people. This is especially true of those who insist on circumcision for salvation. They must be silenced. By their wrong teaching, they have already turned whole families away from the truth. Such teachers only want your money" – Titus 1:10-11
"Watch out for those wicked men – dangerous dogs, I call them – who say you must be circumcised. Beware of the evil doers. Beware of the mutilation. For it isn’t the cutting of our bodies that makes us children of God; it is worshiping him with our spirits." – Phil 3:2-3
"And I testify again to every male who receives circumcision, that he is in debt to keep the whole Law. You who do so have been severed from Christ...you have fallen from grace." - Gal 5:3
"As God has called each man, in this manner let him walk. And thus I command in all the churches. Was any man called in the circumcision [Old Covenant]? Let him not try to become uncircumcised. Has anyone been called in the uncircumcision [New Covenant in Christ]? Let him not be circumcised! Circumcision is nothing. And uncircumcision is nothing but the keeping of the commandments of God. Let each man remain in that condition in which he was called." - 1 Cor. 7:17
"And some men came and were teaching the brethren, 'Unless you are circumcised according to the custom of Moses, you cannot be saved.' But Paul and Barnabas together had great dissension and disputing with these men. . . Then Peter stood up and said to them 'Why do you put God to the test by placing upon the neck of the disciples a yoke which neither our fathers nor we have been able to bear?" - Acts 15:1-2, 7, 10
"But if I still proclaim circumcision. . . then the stumbling block of the cross has been abolished." - Gal 5:11
"I wish that those who are pushing you to do so would mutilate themselves!" - Gal 5:12
Just food for thought. According to the New Testament, you are not a Christian if you are circumcised unless you become 'uncircumcised'. It's not cleaner. It doesn't prevent HIV. It does cut down on UTIs... for 12 months in a population that suffers few to no UTIs (little girls get FAR more). No baby is born with a full grown man penis, so he'll never look like his father (especially since he has mom's dad's genes in there, too) and men don't actually compare penises that much anyway. A little boy is going to care more about the hair than if the tip is pointy or round. It's not even the majority--80% of the world is intact. And finally, just because he doesn't consciously remember it, doesn't mean he doesn't remember at all.
Finally, I know this is a sensitive topic. I am not criticizing anyone's choices, I am simply providing a short bit of information that, on top of my belief that everyone should have a say in their own body regarding cosmetic surgery as well as the fact that the adult penis is easier and safer to alter (and the men get better drugs to kill the pain after, while infants in the first 7 days have less natural pain relief). I have no issue with circumcisions, mine is an issue of consent. I don't agree with infant ear piercing, either.