The concept of insulting a woman because of her sexual habits has to do with the cultural categorization of women. Because most cultures are patriarchal in design (including America), the women in said cultures were thought to be weaker than men, and so, unworthy of roles that required strength, responsibility and authority. As a result, women were given positions of social maintenance, the biggest of which were domestic roles (mother, wife, homemaker, etc.), and this was considered an important job that kept society together: men were the explorers and providers, women stayed at home and kept the social machine lubed up and running.
Since the biggest responsibility women were charged with were breeding a family, it was an almost treacherous act for a woman to threaten the bloodline by having multiple partners: viable birth control is only 60 years old, so a woman keeping monogamous and chaste was a big deal for a long time. Added to which, most Western culture is Abrahamically influenced, so monogamy and feminine subordination was considered a huge religious deal as well. And also since women are not as openly or aggressively promiscuous as men tend to be, it was generally considered throughout ages that a sexually self-possessed woman was not normal, and people were very good at convincing women of this particular myth as well.
So most of the negative shit surrounding women for being "sluts" or "*****s"--different words, but used almost interchangeably--largely has to do with role violation. If women had the same sexual appetites as men, and were allowed the same pursuits, it violates the traditional role responsibility that kept the two genders separate. Since patriarchies tend to define masculinity by its LACK of feminine traits, any semblance of overlap is seen as female weakness contaminating masculine strength. The same thing happened in post-Civil War America: "white men" were "superior" to "niggers" so if "niggers" had the same rights, privileges, and value as "white men", how could you be better then them? To a lot of people, that was seen as devaluing "whiteness" and a lot of that sentiment survives today. Same thing with sex and gender.
So for most traditionalists, how can you define your masculinity/femininity if the two start to overlap?
And since women are far more social by nature than men, they tend to take role position very seriously; if you don't behave accordingly, you're stepping out of line and making the other members of the group look bad. Even if they themselves might sympathize or feel the same way, they won't admit it for the sake of preserving reputation because reputation determines the social value in the eyes of others. That's why a lot of women will attack each other for the same things that men insult them for: they consider it a violation of the natural order, or hierarchy of how genders are supposed to behave and act.
The way I see it, there's preference and responsibility. You can be promiscuous and responsible, and monogamous and irresponsible, and vice versa.
