Sin and disordered inclinations can never be blessed or condoned, the US branch of the "Confraternity of Catholic Clergy", which represents more than 500 clergy, said in a 22 December statement.
Even the appearance of approving a moral evil must be avoided at all costs, otherwise the one who gives the 'blessing' becomes a formal collaborator with evil, the priests wrote.
Blessing a couple, whether they live in a normal or homosexual concubinage, gives the impression "that their relationship is acceptable in the eyes of the Church, which it is not".
A priest is not allowed to bless immoral or disordered behaviour such as abortion, pornography, marital infidelity, child abuse and terrorism, the statement reads.
The Australian and British confraternities have issued similar statements.