Members of Chile's Independent Democrat Union (Union Democrata Independiente) on Thursday will introduce a constitutional amendment that seeks to define marriage as the union of a man and a woman, La Tercera reported.

The move comes 2 days after President Sebastian Pinera sent to Congress a bill that would grant gay and lesbian couples many of the rights and benefits of marriage. The Life Partner Agreement (Acuerdo de Vida en Pareja or AVP) would allow gay and straight couples who have lived together for more than one year to enter the union.

The president signed his bill to the cheers of gay rights activists, but angry leaders of his center-right coalition, which includes the UDI Party, refused to attend the signing ceremony. Pinera, who is fulfilling a campaign promise to back a civil unions bill, told daily El Mercurio last month that his bill seeks to “protect and safeguard” the civil rights of couples living outside of marriage and “safeguard the dignity of those couples, whether of opposite sex or even the same sex.”

The amendment would add to the Chile Constitution language that bans 2 people of the same sex from marrying by inserting “in guaranteeing and protecting the family, only one man and one woman have the right to marry” into the document.

Supporters insist the amendment is necessary to soothe the fears of conservatives who view the AVP as a prelude to the legalization of gay marriage in Chile.