The original assumption that the joke is sexist relies on presuming that the joke makes women into objects used only for sex, but not vice-versa for men. So by

The original assumption that the joke is sexist relies on presuming that the joke makes women into objects used only for sex, but not vice-versa for men. So by claiming that the joke is sexist I think we can reasonably assume the person making the claim believes the engineer and the mathematician to be male.