It’s not, it just says a lot about society and the value people have. There are no equivalents for (white heterosexual) men and you know why? Because they were never really a class looked down upon. There are derogatory words for people of colour, for homosexuals, for (heterosexual) women etc and are/were used to put these people down and show them their place. Since women were viewed as the lesser sex and sexual activity was highly restricted, a way to hurt them and put them down was attacking their sexuality. So yes, calling women a whore is for a lot of people more hurtful than calling her an asshole for example.