Earlier this year, an article in New York Magazine featured a story involving an eighteen-year-old woman who plans to marry and have children with her father. When the interviewer asked her to respond to those who might question her relationship, she offered the following reply: I just don’t understand why I’m judged for being happy. We are two adults who brought each other out of dark places ... When you are 18 you know what you want. You’re an adult under the law and you’re able to consent. Her reasoning is typical of contemporary liberal approaches to sexual morality, which are usually justified by...