But, if the majority of Canadians are, and, want the definition of marriage to stay between a man and a woman, what does it matter??
This misses the point. The whole reason why we have the Charter of Rights and Freedoms is so that the majority cannot decide the rights of an unpopular minority. As the court so majesterially stated in Vriend v. Alberta,  1 S.C.R. 493 at 69:
It is easy to say that everyone who is just like “us” is entitled to equality. Everyone finds it more difficult to say that those who are “different” from us in some way should have the same equality rights that we enjoy. Yet so soon as we say any enumerated or analogous group is less deserving and unworthy of equal protection and benefit of the law all minorities and all of Canadian society are demeaned. It is so deceptively simple and so devastatingly injurious to say that those who are handicapped or of a different race, or religion, or colour or sexual orientation are less worthy. Yet, if any enumerated or analogous group is denied the equality provided by s. 15 then the equality of every other minority group is threatened. That equality is guaranteed by our constitution. If equality rights for minorities had been recognized, the all too frequent tragedies of history might have been avoided. It can never be forgotten that discrimination is the antithesis of equality and that it is the recognition of equality which will foster the dignity of every individual.
As the Court later stated in the Separation Reference,  2 S.C.R. 217 at 74:
First, a constitution may provide an added safeguard for fundamental human rights and individual freedoms which might otherwise be susceptible to government interference. Although democratic government is generally solicitous of those rights, there are occasions when the majority will be tempted to ignore fundamental rights in order to accomplish collective goals more easily or effectively. Constitutional entrenchment ensures that those rights will be given due regard and protection. Second, a constitution may seek to ensure that vulnerable minority groups are endowed with the institutions and rights necessary to maintain and promote their identities against the assimilative pressures of the majority.
So we see two threads: the protection of the rights of a minority matters, and the rights of the minority cannot be subject to majoritarian rule.
While, then, for our reader, it may not matter whether same sex marriage happens or not, for those whom it is deprived, it matters quite a lot. The Courts have said marriage to another person to the exclusion of all others is a fundamental right. That fundamental right cannot be restricted by the will of the majority.