Gay marriage simply ought not to be a matter of political controversy in a civilised society in the 21st century.
It is not just a matter of abstract notions of equality, although they matter, too. There are also strong sociological arguments supporting marriage as beneficial for both individuals and society. It is an institution that fosters social cohesion, brings stability to the upbringing of children, and improves both physical and mental health, as well as longevity and prosperity.
It is, therefore, hard to see why such benefits should be confined to heterosexual members of society. All the evidence is that children raised by gay and lesbian couples have the same financial, psychological and physical advantages enjoyed by their peers whose parents are heterosexual. Instead of clinging to outmoded definitions, our politicians should be advancing the virtues that modern society so badly needs.