... taboos exist because humans are emotional creatures. We feel upset and disgust, and taboos exist to protect us from such feelings. Introducing rape gratuitously into a public discussion upsets some people unnecessarily. Etiquette dictates that we don't do this ... And disgust, like it or not, is the basis for some moral judgments - such as the belief that some things such as human organs or sex be not traded in markets.
Demanding that there be no taboo zones and that reason and logic go everywhere is, in this sense, a demand that people be dessicated calculating machines devoid of emotion. Even if this were desirable - which is very dubious - it is a futile call.
... Dawkins is being inconsistent. What he's demanding is not so much that everyone be dispassionate but that they share his disgust at some things and his lack of disgust at others. He's complaining: "Why can't everyone be like me?" ... In this sense, rationalism is close to narcissism.
People, especially intellectuals, tend to forget how important a role the emotions play in assessing ideas. Almost no one takes an idea on face value as if they were a computer processing a program; emotions and our sense of self are inextricably bound up with the way we process even the most neutral of things.
I suspect a lot of the arguments against taboos come down to an objection against repressive behavior. Don't tell us what to do; people need to have free minds and free lives; etc. What's missing here is any sense of what happens when you break a taboo: you create an emotional reaction, often one of disgust and animosity, and often in the very people you're trying to convince of the unworthiness of a taboo. In short, you're shooting yourself in the foot. (One of the tricks Dale Carnegie taught his followers was that if you want to get somebody to do something, you let them think it was their idea all along, not an idea you're pushing on them.)
I don't think any of this constitutes an argument against rationalism, though. I think it does constitute an argument against the idea that rationalism is always pure. It also continues to convince me that Richard Dawkins, for all of his smarts, is a heartless toad.