80/20 rule is the pareto principle. It's not an immovable number, it's just a commonly observed reoccurring principle in generally every facet of life and nature.
In relation to humans, it refers to the propensity of humans to choose partners who have a higher perceived social value. Typically that value is a public consensus of traits that are desirable in a partner; 6 feet, 6 pack, 6 certs, etc in men. Fit, friendly, and feminine in women. It varies from person to person, but there is a consensus, and it's kinda laughable that people in here would use their personal experience as the exception to the rule to be the standard rule. 80/20 is not a hard-coded number. It's just a concept to illustrate the majority wants the minority.