Dear people who post things like "Incest is neat" and "Whites are smarter than blacks": those things are currently controversial. Therefore, they don't come close to being unthinkable or impossible to talk about.

ADBOC and that's somewhat beside the point, because it seems to me that things are necessarily somewhat controversial to be taboo. As Paul Graham said:

No one gets in trouble for saying that 2 + 2 is 5, or that people in Pittsburgh are ten feet tall. Such obviously false statements might be treated as jokes, or at worst as evidence of insanity, but they are not likely to make anyone mad. [...] If Galileo had said that people in Padua were ten feet tall, he would have been regarded as a harmless eccentric. Saying the earth orbited the sun was another matter. The church knew this would set people thinking.

Now, James Watson and Stephanie Grace might want a word with you. (Larry Summers could file an amicus brief.) Chanting "Racist, racist, cow porn, racist, racist, cow porn" seems to fairly closely match Multiheaded's description that "society would instantly slam the lid on it with either moral panic or ridicule and give the speaker a black mark" for at least some parts of society. This is moral panic and ridicule despite the speaker barely tiptoeing near controversy and hedging everywhere with statements such as:

This suggests to me that some part of intelligence is genetic

I also don't think that there are no cultural differences

I absolutely do not rule out the possibility

I could also obviously be convinced that by controlling for the right variables

I am merely not 100% convinced that this is the case.