LONDON (AP) — The Catholic Church in England intensified its campaign against government plans to legalise same-sex marriage Sunday, urging the faithful to protect the “true meaning” of matrimony for future generations.

In a letter read in 2,500 parish churches across the country during Sunday Mass, the church’s senior archbishops argued that the proposed change would reduce the significance of marriage.

“The law helps to shape and form social and cultural values. A change in the law would gradually and inevitably transform society’s understanding of the purpose of marriage,” Archbishop Vincent Nichols (pictured) and Archbishop Peter Smith said in the letter.

“There would be no recognition of the complementarity of male and female or that marriage is intended for the procreation and education of children,” they wrote.

The archbishops ended the letter by calling on Catholics to fulfill their duty to make sure “the true meaning of marriage is not lost for future generations.”

Britain’s government plans to allow everyone, regardless of their sexual orientation, to have the option of a civil marriage. Prime Minister David Cameron has openly backed the plans, and the equalities minister will launch a consultation later this month on how to change the legal definition of marriage to include same-sex couples.

Human rights activists said the Catholics’ opposition amounted to discrimination.

“They want the law to discriminate against gay couples. Discrimination is not a Christian value,” said gay rights campaigner Peter Tatchell.

Currently only heterosexual couples are permitted to get married in Britain, while civil partnerships, introduced in 2005, are limited to same-sex couples.