Highly educated people tend to have fewer sexual partners on average in their lifetime because quality always trumps quantity.

People with higher education usually have higher standards, better impulse control, and better reasoning skills. Those are traits that prevent ones from falling into a deep hole of casual sex.

Women usually date up. It's the rule of thumb. We know that. This means dating pools for highly educated girls are smaller compared to the rest making them less likely to sleep around. They generally don't want their bodies to be used for pleasure by low class men with a very few exceptions.

It's very possible that a pretty McDonalds cashier to her fun because she has almost nothing to lose while those educated women with prosperous careers do.

Again if promiscuity is one of your main criteria when finding a life partner, education will always be the best filter.