Why do my female friends diss the girls i hook up with?

im a mostly single independent guy and whenever im dating or interested in a new girl they (my female friends) start dissing my love interest. they say she's ugly, she's a slut etc etc when in most cases they have never met her. i'm not like that when they start dating other guys. even if i dont like the guys i just keep my mouth shut. i dont act like a hater. whats wrong with them?