The Wisconsin Supreme Court is scheduled to hear oral arguments on Oct. 23 on whether the state's domestic partner registry is unconstitutional.
The registry grants same-sex couples some legal rights.
Members of a Christian right wing group, the Wisconsin Family Action, filed a lawsuit in 2010 alleging the registry violates a 2006 state constitutional amendment banning gay marriage or anything substantially similar.
Lambda Legal, representing Fair Wisconsin and five same-sex couples, is defending the registry against the challenge.
"The domestic partnership law in Wisconsin is without question constitutional. The limited protections provided by domestic partnerships are a far cry from the significant rights, benefits and responsibilities associated with marriage. To suggest that that the two types of relationships are impermissibly similar is an antigay stretch of the imagination," Lambda senior staff attorney Christopher Clark said in a statement.
A state appeals court upheld the registry last year, noting married couples enjoy many rights that registrants don't.
Last week, the ACLU of Wisconsin filed a friend of the court brief supporting the registry and Lambda.
"The idea that domestic partnerships violate Wisconsin’s anti-marriage amendment is ridiculous," said Larry Dupuis, ACLU of Wisconsin legal director. "Domestic partnerships don’t create a legal status that is identical or even remotely similar to marriage."
The court is expected to hear oral arguments on Wednesday afternoon in Madison.