When do you tell someone that what they just said is offensive?

Inspired by Auggie’s question.

When do you tell someone their view is offensive? I sort of assume (and maybe I shouldn’t, because assuming makes an… anyway…) that everyone on here would say something if someone made some comment about how they seriously thought prepubescent kids were hot. So there is some line where everyone seems to go “Nope, don’t care if you weren’t looking for a debate on the matter, you are going what I have to say”. So, where is that line for you?

Is it how offensive it seems to you, personally? Does if you think they were looking to debate the subject have an impact on if you say something or not? Does it change over topic – for example, are you more likely to say something if someone makes a racist joke than if someone makes a sexist joke? Does the forum matter – are you perhaps more likely to say something at a dinner party, or in a classroom, but Facebook and at work you’re going to keep your mouth shut? Do you take into account if you think someone else there will be offended or hurt by it, but they aren’t saying something (and does that make you more or less likely to speak up)? Do you take into account if you think someone else there will back you up once you make the initial disagreement? Do you feel like you should say something quite a bit of the time, but then you don’t (and why)? Or, conversely, do you say something quite a bit of the time, but feel like you should probably keep your mouth shut more often?

And, when do you think people (all of them, not just you) should just let it go, should not say something?