Sara Cunningham, 55, attends weddings of same-sex couples and volunteers to stand in for parents who refuse to go

As an officiant of same-sex weddings, Sara Cunningham has been to many ceremonies in which couples’ parents refused to attend. So, the 55-year-old mom took matters into her own hands.

In July, Cunningham, of Oklahoma City, Oklahoma, declared on Facebook that she’d be more than happy to stand in for the biological mothers of couples who did not want to go to their children’s wedding.

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“PSA. If you need a mom to attend your same sex wedding because your biological mom won’t,” she wrote. “Call me. I’m there. I’ll be your biggest fan. I’ll even bring the bubbles.”

Cunningham says she received an overwhelming response from people looking for stand-in parents to attend their weddings and graduations. Mothers and fathers from across the country even weighed in, offering to go to the ceremonies to support the couples.

Image zoom From left: Marlee Castillo, Sara Cunningham, and Tabatha Cash Sara Cunningham

“I posted it out of frustration. So often I heard about parents who were not acknowledging these relationships or not attending the wedding. I made that post, not having any idea that it would go viral. But what happened is that, from all over the world, I heard from people … I got some invitations.”

One of those invitations was to Tabatha Cash and Marlee Castillo’s wedding. Cunningham initially planned to officiate the wedding but, after Cunningham shared the post, Cash asked Cunningham if she’d represent her mother at the Nov. 3 wedding.

Image zoom Sara Cunningham

So that’s what Cunningham did.

“My favorite part was seeing Marlee and Tabatha go through this process. They made history that day in their lives. What a beautiful thing to be a part of,” Cunningham tells PEOPLE.

“[Cash] is so beautiful, vibrant, independent, in a relationship that is a healthy relationship. It was bittersweet because I know she didn’t have a mom there. No matter how much she knew she was doing the right thing for herself, there’s still a part of her that’s missing, and that’s her mom.”

Still, there were smiles and tears of joy at the wedding, Cunningham says. And her efforts even caught the attention of actress Jamie Lee Curtis, who sent her flowers that Cunningham gave to Cash and Castillo.

Image zoom Tabatha Cash Sara Cunningham

The mom of two is set to serve as a “stand-in” mom at three weddings next year. But Cunningham says other mothers have already volunteered at three weddings as a result of the post. Cunningham’s good deed comes about two years after she founded Free Mom Hugs, a non-profit organization which provides support and educational services to parents with LGBTQI community.

Although she’s established herself as an ally of the LGBTQI community, Cunningham says that hasn’t always been the case. She says she and her family struggled for years to come to terms with their son’s identity as a gay man. It was through that journey that she learned to accept him and overcome her own biases, she says.

Now, she’s committed to helping other parents on that same journey. In 2014, she published her memoir How We Sleep At Night, which chronicles her relationship with her son and the LGBTQI community. She belongs to an online Facebook group for parents with LGBTQI children (and recommends the private group to moms like her). She also works to end conversion therapy in the state.

Cunningham also took part in a short film about her efforts called The Stand-In.

Along with her efforts, Cunningham says volunteering at weddings is only a necessary part of supporting the LGBTQI community.

I feel “complete joy,” she tells PEOPLE of attending the ceremonies.