The (or at least my, and the majority of people I have encountered who share the same view) argument against infant circumcision is about autonomy not the benefits or losses of circumcision.
Autonomy is the freedom of self. In a medical context this is the right to consent to, or refuse medical actions taken on your body.
Autonomy may be overruled, where a patient is "incompetent" (unable to make an informed decision, for example, such as a baby or somebody with a delusional mental illness, or perhaps somebody unconscious and in need of emergency treatment). But ethically, such an overruling is only considered to be reasonable if the decision made is in that patient's best interest (eg: they will suffer harm unless the decision is made). That best interest must also not be one that could be delayed until the patient has gained competency.
For example, if a patient comes into A+E unconscious and needs a blood transfusion, it will be given. You cannot wait for them to regain consciousness or they may suffer harm.
However, if you noticed that such a patient had a diseased toe, that would probably need to be removed (but without any great urgency), you would not go ahead and do the procedure without their consent. You would (because you could) wait until they were conscious and had given informed consent to remove the toe.
Does anyone here disagree with that as being the ethically correct decision?
Ethics operate in this way for the vast majority of medicine, and it seems to me that infant circumcision is a glaring and unfair exemption.
The ethical basis for children having reduced autonomy is only that they have reduced competency, so I cannot see why there should be an exception for infant circumcision where there would not be one for an adult lacking competency.
You would not circumcise an unconscious adult. You would wait for them to regain consciousness and give consent if you wanted to.
Why then not wait for an infant to grow older and become competent so that they might consent? There is no urgency to circumcision, it can be carried out successfully at any stage in life. It's just like the "diseased toe" on the unconscious A+E patient.
You would not circumcise a 10 year old if they witheld consent. Parents would simply not have the right to do it. Even if that child was gillick incompetent I highly doubt that such a procedure could ever be carried out legally. Because there is simply no urgent benefit to circumcision that could not wait until gillick competency. And a gillick competent child certainly would have the right to refuse.
It does not make sense to exempt babies whose parents happen to feel the strong desire that they shouldn't have a foreskin from the same ethical considerations we apply to everyone else.
People say that infant circumcision is better than adult circumcision but I haven't seen a shred of evidence to support that claim. It seems paradoxical in fact. How can it be preferential to operate on a penis that is not fully formed, cannot be observed in all states, and where you cannot discuss the right amount to be removed with the patient?
Even if there were a benefit, would it arguably be so great as to outweigh the importance of the individual's autonomy? As I see it the benefits of childhood (thus non-consented) circumcisions only outweigh the individuals right to autonomy if there are good medical reasons in that child's obvious best interest to carry out a circumcision.
Adults aren't keen to have their penises circumcised for obvious reasons (it hurts!), but neither are babies and their inability to vocalise their fear and discomfort, even their inability to remember it does not make their post-operative pain less important than if it happened in adulthood.
The only reason to carry out an infant circumcision on a normal foreskin is because parents want you to. There is no medical reason, certainly not one that couldn't wait until that penis belnged to someone capable of saying "no".
And I fundamentally object to the right of other people, whether they are your parents or not, making permanent changes to your body without your permission, because they want to. It should be an individual's choice.
Also, just because something is currently legal does not mean it is right. If everyone held that attitude how would there ever be democratic changes to the law?
If circumcision were not a religious and somewhat culturally normal activity I'm quite sure it would not be legal. If I wanted to have my daughter's clitoral hood removed in infancy I wouldn't be allowed. There is no moral difference, as it stands the law is hypocrisy.
EDIT:
The only thing I can say about legal medical circumcision is that if it were banned, people would only have their babies snipped unprofessionally at at home (many people probably already do do this), which would involve greater pain, trauma and risk. Some babies would inevitably be seriously injured.
So it's more a case of "I wish the awful practice would die out" rather than "it should be banned" because a ban probably would cause more harm than good.