The Alliance Defending Freedom is making its case to defend traditional marriage in Oklahoma, arguing same-sex marriage is bad for children.
The Christian legal advocacy group said legalizing gay marriage would harm children, undermine society, and make traditional marriages unstable.
Lawyers representing an Oklahoma clerk who refused to grant a marriage license to a same-sex couple cited courts and anthropologists in saying children are better off in a home with a mother and a father.
They also added traditional couples would be less likely to marry, or stay together if marriage became a genderless institution not focused on procreation.
Last month, a federal judge said Oklahoma's 2004 ban on same-sex marriages was unconstitutional because the state's voters singled out one set of its residents for different treatment.
The ADF, however, said marriages should be about children, not adults.