According to Reuters, not a single big law firm is representing clients defending state bans on gay marriage.

The news service reported that at least 30 big law firms are representing plaintiffs challenging such laws, while not a single member of the Am Law 200, which ranks based on revenue, is defending state laws limiting marriage to heterosexual couples.

Lawsuits challenging all 31 state bans followed last year's U.S. Supreme Court ruling striking down a key provision of the Defense of Marriage Act (DOMA), which led to federal recognition of the marriages of gay couples. Three cases have been heard by federal appellate court judges.

Law firms representing plaintiffs are also working for free or at cut rates, Reuters reported.

One example cited is the law firm of Akin Gump, which has donated at least 1,100 hours representing two plaintiff couples challenging Texas' ban and filing friend-of-the-court briefs in three separate cases.

Law firms representing defendants or filing friend-of-the-court briefs in support of state marriage bans are predominantly from small law firms or Christian conservative groups, including the Alliance Defending Freedom (ADF) or the Beckett Fund.