So long as an activity is performed in private between consenting adults, it is argued, there can be nothing inherently objectionable about what they do. Why? Because they have given their consent, and consent is what matters most when it comes to one’s decision to engage in sexual activity.
The implications of this position are far-reaching. Many have invoked the consent principle to argue for the permissibility of polyamory and consensual incest. Once we view the morality of sex as being determined only by mutual agreement, then it becomes very hard to make any principled distinctions about the shape of sexual relationships.