"These provisions threaten to plunge the Ukrainian state into the abyss of immorality and sin, to destroy the family as the basic social institution and popularize relationships between persons of the same sex which are unnatural for human beings," the All-Ukrainian Council of Churches and Religious Organizations said in a July 23 letter.
"In its appeals, our council has stressed the need to incorporate, in one way or another, principles of identity and human relations which are traditional for Ukrainian citizens," the council said.
The council, a group of 19 churches and faiths, including Catholics and Orthodox, issued the letter in response to the amendments, which the Ukrainian parliament has been debating. Plans call for adopting the amendments in the fall.
The letter, addressed to Ukrainian President Petro Poroshenko and parliament officials, said two articles covering human rights and civic freedoms, would "seriously threaten" family life and traditional marriage.
Meanwhile, the head of the Ukrainian Catholic Church also criticized the amendments and said the drafting commission, appointed by Poroshenko, had "rejected all warnings" by churches and submitted "opposite proposals" instead.
"References to European experience are irrelevant, because the European Union has opposing views about anti-discrimination legislation, marriage and family, and the legal opportunities for sexual minorities," Major Archbishop Sviatoslav Shevchuk of Kiev-Halych told parliament chairman Volodymyr Groysman in a July 16 letter.
"Poland, Croatia, Hungary and Italy, in particular, demonstrate their commitment to the traditional family and moral values as members of the European Union," the letter said.
The amendments were drafted as pro-Russia rebels continued to hold the eastern region of the country and also have been criticized by Russia for allegedly failing to include enough autonomy for the separatist-controlled Donetsk and Luhansk regions, as outlined in a February cease-fire agreement.
Speaking July 23 to a European Parliament delegation, Groysman said the amendments, currently under review by the Constitutional Court of Ukraine, would "start Ukraine's transformation from a post-Soviet centralized state into a European democratic country."
However, in their letter, church leaders said legalizing same-sex partnerships would be "unacceptable to the moral health of society and its natural development," adding that they hoped to explain their concerns in fresh talks with Poroshenko.
A Soviet-era ban on homosexual activity was lifted when Ukraine regained independence in 1991, although gays and lesbians complain of widespread discrimination and severe harassment in Donetsk and Luhansk.
The country's 1991 constitution describes marriage as the voluntary union of a man and woman, and offers no recognition of same-sex relationships.
Opinion polls have shown little public support for expanded same-sex freedoms, and 25 gay activists were arrested during a brief June 6 gay pride parade in Kiev even though the parade was defended by Poroshenko.