They're not so much irrational as ruled by their feelings. "Feelings" in the sense of things they do not like make them sick. It's either fight-or-flight with them. They can't stand to be in the presence of something they find offensive.
I recently ran across a liberal woman who told me racial jokes made her "uncomfortable." Yet she saw nothing wrong with blonde or Catholic jokes. Hypocritical? Sure.
But why did one make her uncomfortable and one did not? There's nothing rational about her preference. It's all about how she felt.
And she couldn't make the connection, either, even though she has an M.A.
Because of their inability to tolerate these things, they want everything to be censored. So they won't feel "uncomfortable" or made physically ill.
Because words and ideas make them physically ill, they blame their feelings on those words and ideas. Certain words and ideas make them sick, so those things have to be stopped.
Liberals/leftists are dumb enough to talk about "made me sick," "I was ill," etc. And they really are. They have no idea what they are telling people about themselves.
For them it's not about reason. It's all about what's makes them feel ill, no matter how insignificant it is. And at worst, they lose complete control of themselves.
"I don't like you."