6 ideas
Sexual conduct is best understood as the outcome of individuals weighing costs and benefits under constraints, not as the irrational expression of uncontrollable drives. The biological drive sets the demand, but the form sexual behavior actually takes responds to incentives like the cost of search, the risk of pregnancy, and the availability of substitutes.
Attitudes toward sex fall into three families: the moral (sex is governed by ethical or religious rules), the libertarian (consensual sex is private and beyond regulation), and the economic (sexual practices are functional adaptations to underlying conditions). The economic theory predicts that when conditions change, practices change, regardless of stated morality.
As women's earning capacity and economic independence rise, the cost to them of premarital sex, divorce, and forgoing early marriage falls, loosening the norms that once protected female chastity as a marital asset. Sexual liberalization is therefore driven less by changing ideas than by the changing economic position of women.
Marriage shifts from a primarily economic and reproductive partnership toward a companionate bond based on emotional and sexual compatibility as wealth rises and women gain independence. This shift raises expectations of personal fulfillment and consequently raises divorce rates, because the marriage now must satisfy harder-to-meet criteria.
When access to the opposite sex is scarce — in prisons, navies, monasteries, or sex-segregated societies — homosexual behavior among otherwise heterosexual people rises as a substitute. This shows that some sexual behavior is responsive to availability and relative cost rather than fixed orientation alone.
Laws and customs around sex function less by directly forbidding acts than by raising their cost — through stigma, criminal penalties, or restricted access to contraception and abortion. Evaluating any sexual regulation means asking what behavior it makes more expensive and what cheaper substitutes it pushes people toward.