(CNN) -- The mayors of two major cities have opted out of marching in their cities' St. Patrick's Day parades, in what they call a show of support for gay groups that have historically been excluded from the events.

New York City Mayor Bill de Blasio will not march in Monday's parade in Manhattan, and Boston Mayor Martin Walsh sat out Sunday's parade in South Boston.

"I simply disagree with the organizers of that parade in their exclusion of some individuals in this city," de Blasio said at a news conference last month.

"I will be participating in a number of other events to honor the Irish heritage of this city and the contributions of Irish-Americans," he added. The former public advocate did not participate in the parade before becoming mayor, either.

The New York City mayor hosted a St. Patrick's Day breakfast at Gracie Mansion on Monday morning and attended Mass at St. Patrick's Cathedral.

Walsh made his last-minute decision Sunday, the day of the parade.

"I'm disappointed that this year, I will be unable to participate in the parade," he said early Sunday.

"As mayor of the city of Boston, I have to do my best to ensure that all Bostonians are free to participate fully in the civic life of our city," he added.

The South Boston Allied War Veterans Council, which organized the parade, said in a statement on its website that it is not opposed to LGBT (lesbian, gay, bisexual and transgender) people; it just doesn't allow sexual orientation to be displayed.

Organizers of the New York City parade did not immediately return calls for comment.

Three of the country's biggest beer companies have also withdrawn their sponsorships of this year's St. Patrick's Day parades because gay and lesbian groups aren't allowed to march openly.

Sam Adams, owned by Boston Beer Co., pulled out as a sponsor for the Boston parade Sunday, and Guinness and Heineken pulled their sponsorships from the New York City parade Monday.

"We believe in equality for all," a Heineken USA spokeswoman said.

CNN's Blake Ellis and Elizabeth Landers contributed to this report.