Miss Staten Island showed up for the borough’s St. Patrick’s Day Parade on Sunday in her sash and tiara — but was forced to watch from the sidelines instead of marching after coming out as bisexual to The Post.

Twenty-three-year-old beauty queen Madison L’Insalata was banned from participating by parade organizer Larry Cummings on Saturday night, just hours after her sexual preference publicly surfaced, according to L’Insalata and the pageant’s organizer.

“I still wanted to march because I felt I could make a much greater impact being in the parade, waving my rainbow flag,” L’Insalata said Sunday, standing behind a metal barrier along with the rest of the crowd while wearing a rainbow scarf and heart rainbow pin and clutching a little multicolored flag.

Cummings declined comment to The Post.

L’Insalata said the parade organizer banned her and fellow beauty queens who supported her coming out, claiming it was because of safety concerns for them.

Cummings has been dogged by fierce criticism over his repeat exclusion of the Pride Center of Staten Island from marching under its own banner, leading to a slew of public figures and pols bowing out of the parade.

Jim Smith, the longtime executive director of Miss Staten Island Scholarship pageants, told The Post on Saturday that Cummings called him around 10 p.m. to issue the ban on L’Insalata and Miss Richmond County’s Outstanding Teen, Victoria Montouri, 17, for supporting her.

Miss Staten Island’s Outstanding Teen, Angelica Mroczek, and Miss Richmond County, Gabrielle Ryan, already decided to boycott the parade outright because of the gay ban.

“None of them can march,” Smith said bitterly after the late-night call he got from Cummings, adding that he feebly tried to appeal to Cummings. “He was not to be bargained with — you can’t talk to him.”

Cummings cited L’Insalata’s coming out and said she can’t march “for safety reasons,” according to Smith, who added that he doesn’t buy the excuse.

“What can happen to her? I don’t think anyone can harm her. I’m very disappointed, though I’m not surprised. I know they’re very strong in their beliefs,” Smith said of parade organizers.

L’Insalata said she’s shocked by the decision.

“This is definitely a curveball. I was really looking forward to being there and having a discussion, and now there won’t be. It’s sad this had to happen. I thought I was doing something good,” she said. “You want to be part of the change.”

Still, she conceded there can be some “rowdy people” who can make waves at the family-friendly event.

“People can be heated,” L’Insalata said.

“It’s frustrating — I wanted to be in the parade, and it’s unfortunate we can’t have a disagreement and still be in the same place. They’re removing all discussion by not allowing me to be there.”

Still, she doesn’t regret coming out.

“I said what I have to say — I still think that my message got across and that’s most important,” the pageant winner said.