All cultures and sub-cultures have taboos. There are things you just don't do and there are things you just don't say. It has been this way since time immemorial — a by-product of societies trying to keep everyone on their best behavior. Some taboos make a lot of sense. They are there to protect those who need protection and to keep civilization on track. Others are completely irrational. They are simply fashionable opinions gone wild.
Political ones are often the most severe. Here in Ann Arbor, you simply do not express non-progressive views in polite company. You either agree with a left-ish line or you remain silent. At a table full of arch-conservatives and a single liberal, any political discussion would have to be a soliloquy. If you dare vary from this doctrine and you will be deemed closed-minded. This is not surprising considering that Ann Arbor is one of the few remaining places you can still see hippies in their native habitat.
In contrast, my sainted mother, who resides amongst the blue-haired set in Sarasota, lives in constant fear of mentioning her admiration for Barack Obama. If she slips up she can count on any oldsters within earshot to rise groaning from their seats, pocket their Medicare cards, hitch their Social Security-funded green polyester pants up under their armpits, and go off about how the liberals have ruined the country with their meddling. She would then become quite familiar with the sound of antique shop doors slamming in her face.
It's not just politics. There are certain gender issues that have similar sensitivities. For example, whenever you find yourself in social situation with women present, you are tacitly forbidden from criticizing Grey's Anatomy. You may think the dialogue is inane drivel, the characters are shallow clichés, and the plots are insipid pablum — and I'm not saying I do, ladies — but you will keep it to yourself. The consequences to crossing this line are too dire to speak out loud.
In football there are not many taboos, but one of the major ones, outside of Indiana, is that you can't really like Peyton Manning. You may acknowledge his ability and the greatness of his stats, but that's it. You may go no further. In fact, if you are wise, you will instantly qualify any positive comment with, "but when the game's on the line, you gotta go with Brady."