One objection to the Equality of Marriage law is that it redefines marriage. But society must keep redefining its institutions as society evolves so its institutions do not become irrelevant. Marriage is no exception.
In the past 50 years the rise of feminism, the implementation of no-fault divorce and the widespread use of contraceptives -- which has removed procreation as an essential component of modern marriage -- all have redefined marriage. Without this redefinition, marriage indeed is in danger of becoming, if not irrelevant, at least nonessential to modern society.
Society has now redefined marriage as the union of two people in a loving relationship, which the couple proclaims through the institution of marriage. In this loving union, gender is irrelevant. Justice, therefore, demands that society treat same-sex relationships the equal of heterosexual relationships and call their relationship what it is: marriage.