The "ancient institution of marriage" goes much farther back than whatever time period you are referring to -- middle ages, perhaps?
It was instituted by God "in the time of man's innocency" well prior to the advent of property rights and nation states. The commandment "thou shalt honour thy father and thy mother" implies a child's natural right to a mother and a father, a right that is also supported by the basic facts of human biology. And this biological fact is reinforced in the deeply rooted desire of a child to know both their father and their mother.
A child who is told they have two mothers but no dad, or two dads, but no mother, is also being told that this ache in their heart to know their absent dad or absent mother has no value or legitimacy. And that is surely a diminishment of a child's natural right, if not an outright abuse.
And, if you think that celebrating a same-sex union somehow protects a child's rights to equality, then you must be prepared to support polygamous marriage, equally a fact of Canadian life as same-sex unions.
Finally, if two men can be a child's legal parents, then there is no good reason why two men and a woman could not be the legal parents of a child. Once we divorce legal child-rearing from the basic facts of human biology, there is no good reason to restrict legal parenting to two persons.