J. Peter Sartain, the Roman Catholic archbishop of Seattle, on Monday warned Washington lawmakers that gay marriage poses a “grave challenge” to the common good.

According to the Catholic News Agency, Sartain testified against the bill during a Senate committee hearing held on Tuesday.

Supporters of making Washington the seventh state to legalize gay marriage – and the first on the West Coast, not counting California which briefly legalized the union in 2008 – say they have the votes to approve the measure. Opponents have vowed to put the issue before voters.

Sartain said that because “only the union of a man and a woman can generate new life” no “other human relationship is its equivalent.”

The bill, he said, “ignores the origin, purpose and value of marriage to individuals, families and society.”

“Marriage makes a contribution to the common good of society unlike any other relationship, through the procreation, rearing and education of children,” he said. “I urge you to uphold the definition of marriage as the union of one man and one woman.”

(Related: Washington witness says gay marriage would pollute social environment.)