Via the Canadian Press:

The federal prohibition on polygamy is a relic from another time that imposes Christianity on all Canadians and violates their right to religious freedom, says a lawyer arguing against the law.

... [the lawyer] said the Charter of Rights and Freedoms offers two protections regarding religion: laws cannot have a strictly religious purpose, and they can't actively discriminate against people of faith because of their religious practices.

He said it does both.

Even if the law was passed today, under the guise of protecting women and children, [he] said it would still be unconstitutional.