Marriage has a traditional, ancient definition"
Further information: Same-sex marriage
Homophobes might say that marriage is an ancient institution, clearly defined as a union between man and woman. They ignore that Christian and other communities have condoned polygamy, forced marriage[8] and child marriage, and in some cases banned interracial[9] or inter-religious marriage.[10]
Many conservatives claim that extra-marital sexual relationships are sinful. So if same-sex marriage is not allowed, all same-sex sexual activities would be sinful, by definition.
Homophobes might say that gay marriage would ruin the sanctity of straight marriage, without explaining how this would occur. Some use vague Non sequitur explanations as to why it would devalue their marriage For example, "If we recognize counterfeit money, it devalues the real thing. Therefore, if we recognize SSM, traditional marriage is devalued."
Straight people have softened up straight marriage themselves, by allowing divorce, unregistered cohabitation and de-criminalization of adultery. And many years later, when some gays want to marry, they get the blame for ruining straight marriage.