Here is a portion of the Canadian Constiution (The 1982 Act, Charter of Rights and Freedoms) that will/is play(ing) a role in the legalization of same sex marriages in Canada.
I can't seem to find the actual article in the constitution regarding marriage.
Equality Rights
Equality before and under law and equal protection and benefit of law 15. (1) Every individual is equal before and under the law and has the right to the equal protection and equal benefit of the law without discrimination and, in particular, without discrimination based on race, national or ethnic origin, colour, religion, sex, age or mental or physical disability.
Affirmative action programs (2) Subsection (1) does not preclude any law, program or activity that has as its object the amelioration of conditions of disadvantaged individuals or groups including those that are disadvantaged because of race, national or ethnic origin, colour, religion, sex, age or mental or physical disability.