PITTSBURGH—Pennsylvania state lawmakers heard testimony April 10 on a proposed state constitutional amendment that would define marriage as a union between a man and a woman exclusively.

The public hearing at Pittsburgh’s Allegheny County Courthouse was the second of three in Pennsylvania. A similar hearing was held in Harrisburg in March, and another will be scheduled there in the future, said state Sen. Stewart J. Greenleaf (R-Montgomery).

Supporters say the proposed legislation is intended to bolster an existing statutory ban on same-sex marriages in Pennsylvania. They say the institution of marriage is vulnerable to attack, citing the legalization of civil unions in other states.

Bishop David Zubik of the Catholic Diocese of Pittsburgh defended the proposal, saying he attended the hearing to testify on behalf of marriage as one of humanity’s oldest institutions and that married people formed “the very backbone of society.”

“That marriage must be considered truly sacred seems to elude us,” he said. “We have reached the point of a laissez faire view of marriage, a concerted effort to expand its definition so vaguely that marriage essentially becomes meaningless.”

But opponents strongly disputed the proposed amendment, arguing that its passage would be tantamount to institutionalized discrimination.

Pittsburgh City Council President Douglas Shields said the proposed legislation was “wrong for Pittsburgh and wrong for Pennsylvania.” He said the city had a history of protecting its residents from discrimination based on sexual orientation and gender identity.

Shields said the amendment would trump legislation passed in the city 12 years ago that grants domestic partnership rights. That legislation, he said, offered economic benefits by helping local companies attract and retain the best employees.

“We adopted our local anti-discrimination ordinance because we value all families,” Shields said.

Shields and other officials joined representatives of the American Civil Liberties Union and other groups to voice opposition to the bill at a news conference ahead of the hearing.

“This legislation is potentially very damaging to Pennsylvanians,” said Stacey Sobel, executive director of Equality Advocates Pennsylvania. “Not only would it prevent same-sex couples from marrying, it would also potentially prohibit civil unions, domestic partner benefits and more. And that would be for any unmarried couple in the state.”