White Ribbon said it asked John Key about his appearance on The Rock last week, which led to a petition for him to be dropped as an ambassador for the charity.
"We have reached out to the Prime Minister, and we are informed that he did not know what was about to occur, and did not at the time comprehend the rape references or make any. We tke the Prime Minister at his word," the charity said in a statement on its website.
"As a result of this incident, White Ribbon will be writing to all our ambassadors to remind them of our expectations, and the commitment they have made to the campaign. Our and their pledge is to never commit, condone or remain silent about violence towards women. We will continue to try and uphold that."
Last Wednesday, Mr Key was taking part in an interview at the Rock when he was persuaded to enter a cage that had been brought into the studio.
Once inside, he was told by host Tom Furniss to pick up a bar of soap, and did so to laughter from the station's staff.
Picking up soap is a reference to prison rape. Furniss then told Mr Key he had a "pretty little mouth" - a reference to the film Deliverance.
While White Ribbon accepted that Mr Key was unaware of the joke's meaning, it said that defence did not apply to the radio station.
"The Rock radio station created a segment that referenced male rape in a manner that trivialised this horrific violence. It was an awful exercise in bad taste and helped to perpetuate violence by normalising and trivialising it.
"We understand that some people won't see it that way, it will be in their eyes just a joke. We, however, do not agree."
A spokeswoman for the Prime Minister said he had nothing to add to his statement from earlier this week.
"The Prime Minister does these interviews in the spirit of Christmas and the content is decided by the hosts," it said.
"The interviews are meant to be light-hearted, and the Prime Minister hopes the media and the public take them that way."
