I have a grab bag of phrases I use to combat criticism:
1. The adult foreskin when spread out flat is about the size of a 4x6 index card, or it would be if it wasn't cut off at birth.
2. The foreskin contains nerve endings, protects the sensitivity of the glans, and increases lubrication.
3. The circumcision rate is only 33% and I don't want my kid to be picked on in the locker room. (I love being able to turn this one back around now that the trend is going towards leaving infants intact.)
4. Are you going to circumcise your daughter? (Then explain why the two ARE similar, since both are designed to limit sensitivity, both are done without "patient" consent and are thus a violation of rights, and both have make-believe "health benefits.")
5. The number of baby boys that die each year from circumcision complications is similar to the number of baby boys that die each year from SIDS, and I like my baby the way he is--alive and intact.
6. When he's 18, he can make the decision to have it done--with full general anesthesia rather than topical or a painful shot. Once I have it done, we can't undo it.
7. It's too expensive, and it's a waste of taxpayer's dollars. (We have Tricare Prime, military health insurance.)
8. It's easier to keep clean and care for. (I then explain those differences and my personal experiences having an intact son AND a circumcised son.)
9. He will never be the same size, skin tone, or have the same amount/color/texture of hair down there as his dad anyway.
10. I don't want my son's first erection to be a memory of pain and abuse.
11. The rest of the world doesn't think it's necessary, judging by their low circumcision rates.
12. If it's doing such a good job preventing HIV, why does the US have both a high circumcision rate--and a high HIV rate?
13. How often do you cut yourself and then rub poop/pee on the wound?
14. I throw out some random "Did you know?" about circumcision, then leave it at that.
15. "Mind your own business." ;)
Etc... I have a lot of little catch phrases. I throw 1-2 out and explain and then move right along, if possible. It catches them off guard and keeps them thinking.