I would guess that for gentile US boys during most of the 20th Century, being circumcised had to do with a few important cultural conditions, having nothing to do with religion:
The medical culture of their area -- Doctors in most of the country, including overwhelmingly in the Midwest, California and the newer South and Southwest, just circumcised every boy (and charged for it). This was expanded by a post-WWII push to get everyone. Catholics and Protestant families who were missed in the first big medical push in the twenties got included then.
The state of the father -- Once a Catholic immigrant family, or a Protestant rural family had the boys circumcised by the local hospital, there was a tendency to inflict it on each generation so that, "the boy will look like his father."
The collusion of mothers and sisters -- Our American culture of cleaning obsession and germ phobia reached anti-body excesses in the middle century. The "eew" factor of the unfamiliar "must be dirty" body part seemed like a good way to make boys neater and cleaner.
On the other hand, there may have been certain indirect contributions of religion at the earlier stages of the introduction of routine circumcision to the US. The puritan strain of Protestantism had a lot to do with its original introduction in the 1880's -90's to reduce masturbation and "solitary vice." Also, some writers have asserted that, later on, Jewish doctors helped spread routine circumcision (in part and perhaps subconsciously) to make Jewish men less obvious, and thus less subject to pogroms and violent persecution the way they had been in Europe. It had been standard practice to check a man's cock when there was doubt, during the rounding up of Jews.
