Every other sin that a man commits is outside the body, but the immoral man sins against his own body” (1 Cor 6:18 NASB).
Why is sexual sin singled out as uniquely damaging to the body in a way that other physical actions are not? Substance abuse, gluttony, cutting—these are all harmful acts to the body, but they do not do what sexual misconduct does, according to Paul. Typical responses from students to explain this exception are that sex involves the whole person, or maybe because it involves someone else. The same could be said for illegal drug use, so there must be something more.
A theology of the human body indicates that the purpose of the body is for relationship with God, creation, and other people. The body is our bridge to created reality (Francis Schaeffer). Through the body we are vulnerable to pain and threat, and through the body we communicate, respond, work, and experience life in the world.
Sexual misconduct, of the sort that Paul rebukes at Corinth (where the men were going to temple prostitutes), somehow violates and damages the body in a way that other actions do not. The exception seems to be because of the body’s purpose for relationship. Sexual misconduct takes the body and joins it to anther person for a short-term or otherwise illicit relationship apart from the commitment of marriage. The body’s purpose in sexuality is to facilitate a man and woman living as “one flesh.” Sexual misconduct denies that purpose, and cuts the body off from bonding to another. Sexual misconduct disorients, frustrates, and confuses the body from fulfilling its God-given purpose in physical bonding.
Physical bonding through the body is the means that serves the goal of marriage as a “one flesh” relationship. Sexual misconduct separates the means from the end. This devalues the body’s purpose to be of only a little worth in the person’s life (for gratification of desires) and nothing more. This, I think, is why sexual immorality is a sin against one’s own body like no other sin.
The separation of means from the end or purpose in sexual misconduct is like bulimia nervosa, in which the afflicted person separates the means of consuming food from the end of nourishment for the body. With sexual misconduct, a marriage is not served, and the commitment of an enduring “one flesh” relationship is not supported.
The separation of means from ends is also why pornography, masturbation, and sexual fantasy (lust) are harmful. The person indulging in these practices has separated the body’s purpose as a means to relationship from the end of serving that interpersonal commitment of a marriage. These forms of sexual misconduct resemble the binging on food that often accompanies bulimia. Just as these practices of physical bonding make it no longer about committed marriage relationship of “one flesh,” so also bulimia makes the eating of food in a binge not about nourishment at all, but about control and enslavement to appetite.
This does not mean that sexual sin is the worst of all sins, just that it is a sin that particularly damages the body. The designation of sexual sin as the darkest of all sinful categories, perhaps because of this biblical passage, seems to have contributed to unhelpful shame about sexual misconduct, and proper sexual conduct as expressed in marriage.
 Gregg R. Allison, “Toward a Theology of Human Embodiment,” Southern Baptist Journal of Theology 13.2 (2009): 4-17.