The power differential or power imbalance between adults and minors is one justification often given for prohibitions on sex between them. The difference in physical stature and strength alone does not mean there is a lack of consent; otherwise, a petite 18-year-old woman could not consent to sex with a muscular bodybuilder boyfriend. Rather, the issue is mental capacity to give informed consent and the possible existence of a real or perceived custodian-ward relationship. It is sometimes said that children think they have to, or should, do whatever adults tell them, and that they lack the judgment to know any different. Bad judgment is only deemed to prevent consent when the person is either a minor or extremely mentally impaired, though; otherwise, unprotected sex with promiscuous individuals picked up in bars might be deemed non-consensual.
One argument often made is that if a parent were allowed to have sex with his child, the power differential would make it possible to coerce the child into sex. Given the current system, in which such sex is banned, the child can (if he is aware of this option) complain to the authorities if he is being molested or otherwise aggressed against, and get the parent thrown in prison, and himself placed in the care of someone besides that parent (e.g. foster care). Thus, under the current system, it would be incorrect to accuse a parent who has sex with his child of coercing him (unless one views it as a form of coercion to stop providing the child with resources). Libertarians would favor giving children even more options, such as that of entering the workforce, so that he would have more ways to escape abusive situations.
Pat Califia argues that Robin Morgan makes too much of the power differential view, and that her concept of rape comes close to suggesting that virtually all sexual relations -- between men and women, between people of different races, between people of different socioeconomic classes, between able-bodied and handicapped people, and even between people of different size and sex -- are "inherently nonconsensual."
- Rosemarie Tong. Women, Sex, and the Law. pp. 184.