"Don't talk about politics or religion." It's boilerplate advice, especially this time of year when family members and friends with varying cultural outlooks gather to break (if you're a California liberal, gluten-free) bread.
Keeping your opinions to yourself is smart if your priority is conflict avoidance. But keeping the peace makes for seriously boring holiday meals.
Aside from the tense tedium of forced blandness, all that self-censorship accomplishes is to paper over conflicts and differences everyone knows or suspects are there anyway. Nothing gets resolved.