In response to the assertion that marriage should be exclusively defined as between a man and a woman, here are some counterarguments relevant to Canada:

Legal Equality: Denying same-sex couples the right to marry is discriminatory and creates a second class of citizens. Legalizing gay marriage ensures equal access to benefits and protections enjoyed by heterosexual couples.
Changing Traditions: The concept of “traditional marriage” has evolved over time, and historical accuracy does not support the idea that marriage has always been strictly between a man and a woman.
Gender Definition: Insisting on defining marriage based on gender begs the question of how gender itself is defined. Supporters of gay marriage argue that gender should not be the sole criterion for marriage.