Since mine are girls, I don't often get asked this question.
But, when circumcision comes up in conversation, my typical response is:
1. We didn't really know whether we wanted to or not, but our Pediatrician told us that she won't do them unless the parents have a religious reason, and that the AAP doesn't recommend circumcision.
2. I did a little research, and found she was right. No medical organization recommends circumcision. I figure, if we're born with them, we must need them! Why put a baby through a circumcision when it's not medically necessary?
3. (Since I live in a rural area) - Also, we don't circumcise our animals - so if circumcision is so important from a hygiene perspective, why don't vets circumcise all the breeding bulls/stallions etc. here? After all, we humans have access to soap, clean water, it's easy for us to clean with our hands, unlike the animals we aren't circumcising..... *This comment usually really makes people stop and think - for whatever reason, the 'cleanliness' argument seems to be the big one I hear around here.*
I've had this discussion with my older sister (who was passionate about circ from a 'cleanliness' perspective). She wasn't defensive at all - she said, "Well, I might have made a different decision if this information had been out there when G was born!" I think that the "My doctor says...." followed by "no medical organization recommends it" approach is a good way to defuse the defensiveness, at least if the kiddos are a bit older. In this way, mom (because it's usually a mom asking this) doesn't feel so defensive, AND has the "up to date" information in case circumcision comes up in a conversation she has with someone else (of course, the AAP hasn't recommended routine circumcision for a long time but they don't have to know that).... I often compare it to breastfeeding/formula-feeding; that doctors gave the wrong advice on that for a long time, too, and even now that they 'know better,' they still don't always give the best advice/support, just as some doctors continue to push circumcision even though their medical organizations don't recommend it.