Some girls may object to being called "sluts" or "whores" because of the connotations of the word. "Sluts" and "whores" do not get treated well and are not viewed positively. Even when substituting a different, less loaded word, the fact remains that we, societally, view sexual promiscuity as MUCH more permissible for men than for women.
Additionally, individuals' definitions of what constitutes being promiscuous vary WIDELY. A woman might think she's not "slutty" for having slept with only four guys in her lifetime, whereas someone else might consider her "slutty" for having slept with more than one.