The backlash has even taken the form of threats on the couple's lives, Engelhart told the Hollywood Reporter.
"People have taken up the mob mentality," he said. "It saddens me that the choices we made in the privacy of our home would lead people to feel so betrayed that it's elevated to threats on our lives. I'm very discouraged."
The blog that sparked the campaign featured Terces Engelhart announcing that the couple had cooked their first hamburgers after abandoning 40 years of vegetarianism and 11 years of veganism. Adding insult to injury for committed vegans and loyal restaurant patrons was a photo of her husband chomping into a burger inside a roll and pictures of meat hanging in a fridge.
Explaining his volte-face on eating meat, Engelhart wrote: "We need cows to keep the earth alive, cows make an extreme sacrifice for humanity but that is their position in God's plan as food for the predators.
"Sacrifice is part of life. As in the passion of Christ, we all have to spill our blood for humanity to know the Father. The cow's sacrifice was been ordained, ours we must choose."
Campaigners say the couple's decision is a betrayal of veganism's "belief system". "You are patronising a restaurant that you think has that philosophy, and it turns out it doesn't," Carrie Christianson, founder of the Facebook boycott page, told the Hollywood Reporter. "Vegans should know that this restaurant has a farm that slaughters animals."
The chain has established a widespread reputation and attracted celebrity customers including Gwyneth Paltrow, Beyoncé and Sacha Baron Cohen since the first Cafe Gratitude opened in 2004.