A pastoral letter agreed at the US Conference of Catholic Bishops warns about the dangers of same-sex marriage and partnerships, saying they will affect everyone.

The letter restates the church’s opposition to gay rights, contraception, cohabitation and divorce.

It says that legal progress to recognise gay relationships poses “a multifaceted threat to the very fabric of society, striking at the source from which society and culture come and which they are meant to serve.”

The letter continues: “Such recognition affects all people, married and nonmarried: not only at the fundamental levels of the good of the spouses, the good of children, the intrinsic dignity of every human person and the common good, but also at the levels of education, cultural imagination and influence, and religious freedom.

“To promote and protect marriage as the union of one man and one woman is itself a matter of justice. In fact, it would be a grave injustice if the state ignored the unique and proper place of husbands and wives, the place of mothers and fathers and the rights of children, who deserve from society clear guidance as they grow to sexual maturity.”

The letter also describes contraception as “objectively wrong” and “essentially opposed to God’s plan for marriage and proper human development”.

Couples living together who are not married are warned that cohabiting “involves the serious sin of fornication. It does not conform to God’s plan for marriage and is always wrong and objectively sinful”.

The letter is seen as guidance for Catholics, especially those about to marry or facing problems in a marriage.

Some bishops had expressed concerns about the tone and content of the letter.

One part, calling contraception and and cohabitation “intrinsically evil” was toned down and replaced with milder language.

This week, research commissioned by the church found that gay priests were no more likely to sexually abuse children than heterosexual ones.