The two main arguments stated by proponents of circumcision are fallacies.
The hygiene argument is clearly spurious because it then would most likely have to apply to females as well. No woman would suggest to cut off part of their genitalia because it is "cleaner". How about soap and water?
The religious argument is unfounded as well. Just because a religious belief has been held since antiquity, it does not have to be continued for ever. A certain Galileo nearly lost his skin when he opposed the Catholic Church by maintaining that the earth was not flat. If religions want to sustain their relevance they have to accept that human rights are the basis for peaceful communities in the 21st century. Circumcision of newborn or adolescent boys is an act of mutilation because it disregards the rights of the child. If grown-up men are willing to undergo circumcision to follow their religious commands, they may well do so.