The costs of engaging in sexual activity have fallen dramatically over the last half century (AIDS notwithstanding),
With less and later marriage and more divorce, women spend less of their sexually active years married and so their demand for nonmarital sex- sex made in any event less risky by improved contraception and the availability of abortion- soars.
In communities (and there are still some) in which premarital sex is strongly disapproved, young people marry to have sex, but marriages so motivated are likely to end in divorce, producing more unmarried people and so more demand for nonmarital sex.
In addition, with the decline of the farm population and the rise of social security, children's value as farm labor and old-age insurance diminishes, and as a result the demand for children falls.