The reigning football and baseball champions, along with baseball's small-market Tampa Bay Rays, are among the thousands of businesses, religious groups, advocacy organizations and politicians who are filing legal briefs at the U.S. Supreme Court in support of gay marriage.
The cases from Kentucky, Michigan, Ohio and Tennessee will be argued April 28, and a decision is expected by early summer.
Roughly six dozen briefs backing pro-gay rights plaintiffs in the four states are expected by the Friday deadline. Included is a "people's brief" filed by the Human Rights Campaign with the signatures of 207,551 people.
The Super Bowl-champion Patriots, the World Series-winning Giants and the Rays are part of a brief from hundreds of U.S. businesses.
The Patriots play in Massachusetts, the first state to allow same-sex couples to marry, and the Giants represent a city that is notable for its lesbian, gay, bisexual and transgender community.
Rays president Brian Auld said it was important that his team stand up as well.
"We're a small but visible business, and I actually think it's important that we send this signal of inclusion to the entire region," Auld said in a telephone interview Thursday as he watched the Rays' first spring training game in Port Charlotte, Florida.
The team also has participated in the "It Gets Better" project to encourage LGBT teenagers who have been bullied.
"Our players have traditionally been supportive of these kinds of things," Auld said.
The Patriots, Giants and Rays are taking a stand the same week New York Mets second baseman Daniel Murphy made headlines for comments on MLB ambassador for inclusion Billy Bean, who is gay. "I disagree with his lifestyle," Murphy told NJ.com on Tuesday. "I do disagree with the fact that Billy is a homosexual."
A team spokesman said Wednesday that Murphy will no longer address his religious beliefs and will stick to baseball.
ESPNNewYork.com's Adam Rubin and The Associated Press contributed to this report.