"When we advance the idea that some people should be treated differently because of who they are," they wrote, "demeaned in public as lesser beings, not worthy of the same rights and benefits as others despite their actions as good citizens and neighbors, then we deny them equal protection under the laws. America has walked this path before, and courageous people and the Court brought us to the right result. We urge the Court to repeat those actions here."

Ayanbadejo, whose mother is white and father is black, sees a similarity in the anti-miscegenation laws that would have prevented his parents' marriage and the effort to legalize gay marriage.

Kluwe and Ayanbadejo have been vocal supporters of similar measures, but this signals a more formal collaboration on the issue.

"We basically bounce ideas off of each other," Ayanbadejo said in an email. "We will be spending some good time together this offseason, not only in cyber space but also in person."

Kluwe and Ayanbadejo have spoken out against intolerance in the NFL, where players occasionally will make homophobic remarks such as 49ers safety Chris Culliver's comments during the Super Bowl. Kluwe previously wrote a passionate and profane open letter to an elected official who had opposed Ayanbadejo's support for marriage equality.

"We have yet to have had our first disagreement," Ayanbadejo said of Kluwe. "Actually, I take that back. One day I was venting to him about how ignorant some of our NFL colleagues are and he encouraged me to relax and give them a bit more credit, as it will be them that carry the torch that Chris and I hand them."