Women are called sluts based on the perception of their willingness to have sex with strangers or multiple partners.
This goes back to before the sexual revolution of the 1960's. Women were expected to be chaste. Society saw a pure woman as someone of virtue. This lined up more with a Christian view of the world. As society diverged from Christianity, the morals still have their influence as well as the desire to see women as virtuous.
When the sexual revolution hit, the stigma of sex outside of marriage was being removed (and is mostly gone now). This along with increased availability of contraceptives and abortions make it so women no longer have to fear unwanted pregnancies.
The catch to all this is found in evolutionary biology. Because women have an evolved nature to nurture young, there is still the craving for a committed relationship. Meanwhile, men have evolved with the goal of spreading seed. From an evolutionary standpoint, the most successful strategy is to sleep with a new partner every night. With the removal of the Christian ethic (which pushed for lifelong monogamy), men are seen as merely fulfilling their biological id.
So, it's a combination of latent Christian ethics and recognition of evolutionary traits that leads to women who give it away being seen as sluts.