I think circumcision is ok for Jews and some other nations who descend from Abraham, alav haShalom. But I oppose it for other Gentiles, except for real medical reasons. I think a true Noahide should not do it, since Abraham who was under the Noahide Laws didn't get it until he was ordered by Hashem.
But the "convert" who questioned it for Jews......hmmmmm if he thought like this at the moment he accepted conversion......he would be rejecting a Torah pasuk and his conversion could be invalid.