The nation's Roman Catholic bishops have affirmed that the church defines marriage between one man and one woman, and sex is meant for procreation.
The pastoral letter was issued Tuesday in Baltimore by the U.S. Conference of Catholic Bishops.
The bishops said they are disturbed that a growing number of people view marriage as a private or individual matter, instead of an issue critical to building a healthy society.
They said in the pastoral letter that redefining marriage to allow same-sex unions would damage the common good and would ignore the proper role of husbands and wives.
Meanwhile, in a preliminary report commissioned by the bishops, a study found no evidence that gay priests were more likely to abuse children.
Researcher Margaret Smith from John Jay College of Criminal Justice said the study so far has found no connection between being gay and an increased likelihood of abuse.
Questions about any link had been raised widely in the church because most of the known victims were boys.
Gay advocates and many experts on child sex abuse have long held that there is no link.
The preliminary findings were released Tuesday at the U.S. Conference of Catholic Bishops meeting in Baltimore.
The study is looking at what led to the clergy sex abuse crisis. The final report is expected at the end of next year.