It has been shown that as an individual moves through the stages of excitement, plateau and orgasm in the sexual response cycle, thresholds of pain increase. This could be seen as evidence of the fading of the outside world during the sexual response cycle. It has been thought that rigorous training in the disciplines of meditation, self hypnosis, ect. When one looks at the similarities between trance states and intense sexual experiences, one realizes that the average individual may achieve highly pleasurable trance states several times during a normal lifetime. It is assumed that deeper trance states can be achieved when sex occurs in a healthy, monogamous relationship.
Casual sex is likely to occur within a lighter, more surface level trance.
Monogamy may not only be a safer alternative to multiple sex partners, but may also lead to more absorbed, pleasurable, and meaningful sexual experiences. Psychological treatment procedures such as sensate focus and hypnosis may be useful in increasing absorption in the sexual trance. For more information on sensate focus, see your local sex therapist or Seven Weeks to Better Sex by Domeena Renshaw. For more information on trance states, the more research minded may enjoy Altered States of Consciousness edited by Charles T. Tart. A more leisurely read on the subject would be Journey to Ixtlan by Carlos Castaneda.