Humans have been compared to other species in terms of sexual behavior.
Neurobiologist Robert Sapolsky constructed a reproductive spectrum with opposite poles being tournament species, in which males compete fiercely for reproductive privileges with females, and pair bond arrangements, in which a male and female will bond for life.
Generally, during much of recorded history of humans in civilization, and into the Middle Ages in Europe, weddings were seen as business arrangements between families, while romance was something that happened outside of marriage discreetly, such as covert meetings.
From about 1700 a worldwide movement perhaps described as the "empowerment of the individual" took hold, leading towards greater emancipation of women and equality of individuals.
The reasons older men dating younger women have less to do with sex and everything to do with a profound desire to reassure ourselves that we've still got "it." "It" isn't just physical attractiveness; "it" is the whole masculine package of youth, vitality, and, above all else, possibility. Age difference in relationships is no longer important as long as two people find something in common.