The trial of a challenge to Michigan's ban on gay marriage opened this week in downtown Detroit and is expected to last until next Thursday.

Plaintiffs in the case are a lesbian couple looking to jointly adopt their three foster children – Nolan, 4, Jacob, 3, and Ryanne, 3 – but who cannot because of the state's 2004 voter-approved constitutional amendment limiting marriage to heterosexual couples.

At the suggestion of U.S. District Judge Bernard Friedman, April DeBoer and Jayne Rowse of Hazel Park amended their lawsuit to also challenge the state's marriage ban.

According to, attorneys representing the plaintiffs have called five expert witnesses to the stand.

On Friday, Nancy Cott, a Harvard professor with expertise in the history of marriage, testified that neither procreation nor gender roles are a requirement for marriage.

“Nobody applying for a marriage license has ever been asked what roles the two parties will take on,” Cott said.

“Inability to have a child has never been grounds for divorce in any state,” she later said.

Marriage in contemporary America, Cott testified, is “about mutual support in an emotional way as well as in an economic way. … So that the relationship of marriage to gender has been on a historical trajectory toward a kind of neutrality.”

Witnesses for the state will take the stand on Monday, including Mark Regnerus, an associate professor of sociology at the University of Texas at Austin, who has been widely criticized for concluding in a 2012 study funded by a conservative think tank that children are negatively affected by having gay parents.

(Related: Mark Regnerus: Gay marriage will embolden straight men to demand open relationships.)