As anyone who is opposed to circumcision can attest, people who favor circumcision often make desperate and easily refuted arguments. Is there really anyone who thinks "Because he should match his dad" is a valid argument for genital mutilation?
The purpose of this post is not to spend time refuting each and every argument in favor of circumcision. For that, I recommend this episode of Penn and Teller's Bullshit on the subject. Rather, I want to discuss what I think are the reasons for the often fevered and emotional defenses of the practice.
Reason #1: You have a child that is circumcised.
Consider the following; you've spent the last 5 years doing the best job you can raising a child. You have his best interests at heart, and you love your child dearly. When he was born, you circumcised him because your doctor told you that it was medically advantageous and would benefit your child.
Now some guy on the Internet tells you that circumcision is abusive and amounts to barbaric genital mutilation. What kind of emotional reaction are you going to have? There is a lot of shame involved here. It takes a strong, emotionally stable person to be able to digest what has happened here. Every parent wants to think of themselves as a "good parent", so the notion that you mutilated your child's genitalia is going to be a lot to swallow.
Reason #2: You yourself are circumcised.
But maybe you don't have kids. You yourself are circumcised, and you've never felt worse for the wear for it. Everything seems functional and pain free. Sex is pleasurable and fun. Your parents seem like nice people who wanted what's best for you, and they were told that circumcision was a good idea by your family doctor.
Now some guy on the Internet tells you that circumcision is abusive and amounts to barbaric genital mutilation. What kind of emotional reaction are you going to have? Accepting this claim amounts to believing that your genitals are deformed and unnatural. It also means accepting that your parents had you deformed on purpose. Most people are strongly committed to the idea that they had "good parents", and this development certainly puts a thorn in that notion.
Reason #3: Someone you love or care about is circumcised or has had their child circumcised.
I won't go through the whole rigamarole again. I think you get the gist of it at this point.
Essentially, there are a lot of emotional barriers than opposing circumcision runs up against. Someone has to be willing to question their own value and worth, as well as their moral value and the value of their parents, in order to seriously consider the idea that circumcision is barbaric genital mutilation. Most people are not willing to do this.
That being said, of all of the various abuses parents subject their children to, circumcision actually isn't that bad, particularly in a culture where it is commonplace. But "not that bad" is not the same thing as "good" or "morally acceptable." Stealing is still wrong, for example, even if it is not as bad as rape.
Your best bet, if you're going to discuss circumcision with someone who is likely to respond poorly to the idea, is to bear in mind the emotional cost that seriously considering the idea will have on them. Don't shame them or beat them over the head with the idea. And don't expect immediate agreement; few people change their views after one discussion on a subject.