It might seem harmless, but the purpose of a joke is to draw a distinction between those who get it and those who don't, fostering a sense of community. In this "joke", the in-group is people who don't know something; the community ideal fostered there is that knowledge is undesirable, that anything that seems unintuitive to the uninformed mind is inherently ridiculous. The "joke" has no effect if it doesn't do this. Entertaining the idea without challenge is dangerous.