Discussing a controversial issue is only possible with mutual respect. Saying an idea is stupid (which is a statement of opinion and not fact), is not the same as saying someone who believes in or agrees with said idea is stupid. Since the people you are dealing with have made a point of ranting about those who disagree with their viewpoint, I wouldn't have high hopes for a productive discussion with them.
If they are disparaging a group of people that you fall into, you get to decide whether you are comfortable basically sitting and listening to them (unknowingly) insult you. If you are, by all means keep silent. If you are not, you might say "What an interesting assumption." or perhaps if you are feeling more combative you might put them on the spot by directly saying "So if I understand you, anyone who thinks B about X is stupid. Do you think I'm stupid? Because I happen to think B about X." Unfortunately, depending on who you are talking to and their relationship
with you, they may very well respond "Well, I guess you are
[stupid if you really think B about X]!"
If you don't have any interest in getting into a debate or getting browbeaten for your opinions, then I don't think you are rude to say lightly but firmly "No actually I don't" when asked something like "Don't you think people who believe B are idiots?" It doesn't mean you agree with people who believe B, just that you don't feel like sitting and listening to your friends and family bash whatever group they disagree with. It doesn't sound like a fun way to pass the time.
Of course if they are saying things like that, they are probably looking for a
argument and will come back with "What? Why not? It's true! You
don't believe B do you?"
So the question is: to engage or not engage. What will either accomplish? What will you get/how will you feel if you don't speak up? If you do? Do you want a debate? Do you just want them to stop talking about A opinion regarding <controversial topic X> like it's a forgone conclusion instead of their biased opinion? Do you want them to stop bashing others or shut up about <controversial topic X>?
IMHO, I don't think you should allow them to hold forth with such rude ranting if it is bothering you enough to ask about it on this forum. How exactly you handle engaging, or distancing, yourself regarding this issue will have to be your choice. If you do and say nothing, be prepared to have your boundaries crossed and the assumptions and onslaught of opinions continue.
Perhaps we need another tool besides a Shiny Spine - something like a Silicone Shirt(R) that everything just runs right off of.
(What else is nonstick/water proof?