The government has called the proposed law the "Zero Tolerance for Barbaric Cultural Practices Act." It would make it illegal for anyone under 16 to get married. It would also explicitly require consent for marriage, block anyone in a polygamist relationship from immigrating to Canada and allow for a peace bond to prevent someone from participating in forced or child marriage.
The law would also make it illegal for someone to remove a child or non-consenting adult from Canada to have that person married.
In an interview to air on CBC News Network's Power & Politics on Friday at 5 p.m. ET, May says the legislation is fine, although it mostly aims to "make illegal things that are already illegal," like polygamy and kidnapping.
"The problem with the bill is the bumper-sticker title. Zero tolerance for barbaric cultural practices? I think it's to fan the flames of a Conservative base that's disillusioned with the lack of ethics of the prime minister. So they're trying to rally round their base by saying we're here to stand against barbaric cultural practices," May told the CBC's Rosemary Barton.
"People won't know what's in the bill. Goodness only knows how they'll fill in the blank of what they think a barbaric cultural practice is. I think it's appalling."
Other critics have noted the offences listed in the proposed legislation, known as bill S-7, are already covered by the existing Criminal Code.
'Wasting our money' on niqab appeal
A few measures in bill S-7 tackle activities that aren't already illegal, and May noted she agrees with those changes.
May also criticized the Conservatives' appeal of a Federal Court decision that would let citizenship judges allow niqabs during citizenship ceremonies. May said if a woman in a niqab is prepared to prove her identity in private with a female official, that is sufficient.
"I'm with the Federal Court on this. And I think Harper's frankly wasting our money once again on an appeal that the federal government will lose."
May said the debate over the niqab "creates a very hostile, emotionally charged and difficult environment" that makes it harder to work together to prevent radicalization.