A redefinition of marriage in law "is not and never will be inevitable," he said, but these threats require constant vigilance and effort.
Bishop Cordileone also pointed to a new social science study conducted at the University of Texas at Austin.
Entitled "The New Family Structures Study," the survey examined "a very large, nationally-representative and random sample of American young adults – ages 18 to 39 – who were raised in different family or home environments," including with same-sex couples, single parents, step families, adopted families and married biological parents.
The study measured various areas of wellbeing, including social and economic condition, psychological and physical health and sexual identity and behavior.
In 25 of 40 areas measured, there were "significant statistical differences" between young adults raised with their married biological parents and those raised in other situations. And "in no area were children better off in an alternative arrangement."
"As this new study indicates, social science continues to affirm that children thrive and do best with their mother and father in an intact home," said Bishop Cordileone.
The new study is being recognized in the scientific community for its "comprehensive and scientifically respectable approach," he added. The researchers themselves disagreed about family structures, but agreed to work together on an objective study.
The bishop said that the study can be "a benchmark for further studies and findings." It also calls into question previous studies that were smaller and more restrictive, often based on
self-selecting samples that were not random, he observed.
In working to defend marriage, he noted, the conference should keep in mind young people, particularly those who have seen or experienced the effects of being raised in a broken family and are yearning for a better way.
"The protection of marriage as the union of one man and one woman is a work of justice and is foundational to the good of all," he said, "especially for those most vulnerable among us, our children."