The legal decision represented an about-face for Nevada, which just last month filed arguments in the 9th U.S. Circuit Court of Appeals case defending a gay marriage ban approved by voters in 2002.
Masto called the arguments "no longer sustainable."
The American Civil Liberties Union of Nevada hosted the events for a campaign they called "Freedom Nevada," along with the Progressive Leadership Alliance of Nevada and Human Rights Campaign nonprofits, and the Freedom to Marry Inc. lobbying group.
ACLU Executive Director Tod Story said the aim was to begin sharing real experiences of loving gay and lesbian couples "and show why marriage matters."
Jeff Garofalo, a lawyer and self-described conservative Republican, was one of 17 supporters during the news conference at the Grant Sawyer state office building in Las Vegas.
Some held signs, in English and Spanish, calling for same-sex marriage rights.
Garofalo pointed to what he called a varied "checkerboard of rights" from state to state, and called marriage a basic freedom.
"As a matter of policy, long-term committed relationships should be encouraged," he said.
Longtime couple Ron Virtue and John Jaworek of North Las Vegas noted in a statement that Valentine's Day is Friday, and said they saw a glimmer of hope that they'll be able to solidify their lifetime commitment and be legally recognized as a family.
Bishop Dan Edwards, head of the Episcopal Diocese of Las Vegas, called freedom and equality basic values, while the Rev. Rick Millsap of Trinity Episcopal Church in Reno said Nevada shouldn't single out one group of people for unfair treatment by not letting them marry the person they love.
"We are taught to treat others as we would want to be treated ourselves," Millsap said.
Organizers noted the federal government, other states and courts around the country have chipped away in recent months at laws that prohibit same-sex marriage and benefits.
Reno Mayor Bob Cashell said at a news conference at the Trinity Church parish that tourism could benefit from support of gay marriage.
"To be competitive we need to continue to welcome a diversity of business to our great state," Cashell said.
"For somebody who has been married 49 years," he added, "I can't imagine someone telling me I couldn't marry the person I love."