Over this past Thanksgiving holiday, Timothy Craig Braun and Peter David Arnold broke the news to their 19-year-old fraternal twin daughters: After 25 years together, they were finally getting married.

It was a surprise announcement since the couple, who live in Montclair, N.J., had always told their daughters that they were already married.

“I was overcome with emotion and super happy for them,” said Beatrice Braun-Arnold, who is a sophomore at Denison University in Ohio. She said that the family had some open conversations in the following days, leaving them all feeling better. “It was an eventful Thanksgiving, to say the least,” she said.

Her sister, Liliane Braun-Arnold, a sophomore at Smith College in Massachusetts, said she “had an idea” that her fathers were not married. “They were very adamant about being each other’s partners, not each other’s husbands,” she said. Furthermore, when New York legalized same-sex marriage in 2011, she said, “I overheard my parents talk about if they should get married.”

Indeed, when Mr. Braun, 61, an executive producer who owns Braun Production, a media training and video production agency, and Mr. Arnold, 65, the executive director of the Fashion Scholarship Fund, met in 1998, legal marriage wasn’t even an option for same-sex couples.