It is likely that the Canadian Government took the gamble of this approach in order to avoid the hassle of obtaining the agreement of the Provinces while banking upon the likelihood that no one would have the standing or motive to challenge it. Moreover, if the Duchess of Cambridge has a first-born son, it will avoid the problem of having a female monarch of the United Kingdom and a younger brother who becomes the monarch of Canada. Hence, the chances of getting by with such a constitutionally shoddy arrangement are reasonable.
Nonetheless, it shows a disappointing lack of understanding of the Crown and its divisible nature and a willingness on the part of Canadian politicians to sacrifice Canadian independence to avoid having to engage with the Provinces.
Update 12:42pm. And to all these concerns about the nature of our country, the House just shrugged and agreed to pass the bill unanimously at all stages.