Women are more promiscuous today than 100 years ago, that's for sure. And even more promiscuous than 200 years ago and alot of that has to do with people marrying at a far older age than yesteryear. In 1920, the median age of women that married was 21 years old. Today, the median age of women that are married is now 28 years old. Live longer as a single person, you are bound to have more sexual partners.
Outside of just age, the amount of people we meet and the potential number people we can meet today far supercedes what was possible in years past. Young people can travel outside their towns, cities, counties, states, and even to other countries. The potential to meet someone is just vastly different today but with that said, nothing compares to the 50s in terms of promiscuity for women. And that's not even taking religious and acceptible societal norms surrounding sex in American culture has changed so much. Decades and hundreds of years ago, purity was seeked and a virtue and both men and women knew and understood it. This is all speaking in general of course but that was the attitude 100+ years ago. Today, women can talk about sleeping with others without a care so of course promiscuity will only increase.