The causalities of this arrangement were children who experience split homes or were raised with an absent parent, he said.
The normalization of same-sex unions "teaches that mothers and fathers are replaceable in terms of parenting" and will likely lead to an increase in children who do not know at least one of their biological parents, he observed.
Girgis critiqued "the main vision of marriage" espoused within society that defines marriage as primarily an emotional union.
He explained that while it is consistent for that view to accept same-sex partnerships as marriage, that view is unable to explain "less controversial features that we all agree set marriage from other bonds," such as monogamy and exclusivity.
Furthermore, he said, in making emotion the determining characteristic of marriage, there is no reason for it to require a "pledge of permanence," and there is no logical reason to prevent marriage from being extended to multiple partners or non-sexual partners who share an emotional bond.
Instead, Girgis suggested a definition of marriage based upon "complementarity," or the ability of spouses to bear and raise children, saying that this definition of marriage with the family at its core explains other attributes associated with the institution, such as permanence, monogamy and the sexual relationship of the spouses.
He said that the common contemporary understanding of marriage "suggests that the norm of sexual complementarity is arbitrary," but if one accepts that a man and woman's ability to bear children is unnecessary for the institution, "then so is permanence, so is monogamy."
In addition, Girgis commented on the appeal of redefining marriage as a way to combat anti-gay bias and unjust discrimination against those who have same-sex attractions.
While he agreed that injustice must be countered, he warned against using marriage to do so, cautioning that such a move would have devastating harmful consequences.
Rather than changing the definition of a timeless and foundational social institution, he said, "I think the answer to bullying is to fight bullying, the answer to prejudice is to affirm the equal dignity of every human being."