It isn't right to disparage anyone for who or what they are, and it is a double standard, but let's be honest: a woman making a sexist comment towards a man, a black person speaking ill of white people, a gay person being salty about straight people, or an overweight person criticizing a someone's being skinny holds significantly less impact than the reversal. They're criticizing things that the other person is usually celebrated/privileged for (and most likely it's out of the frustration that they have felt for being treated less than for not being these things themselves), and thus their negativity has less sting and power to degrade.