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A person who is sexually aroused by the foreskin (as opposed to merely male genitalia) is said to have a foreskin fetish. Like all sexual fetishes, in some cases this may interfere with normal sexual or social functioning.
Foreskin fetishism can be clasified as a paraphilia, characterised by intense sexual urges involving (in this case) the male foreskin. The foreskin fetishist can be male or female, although indications from Internet pornography offered are that it is a predominantly gay male interest.
While the term may be used informally, a clinical diagnosis must meet formal criteria which include distress or impaired social or sexual functioning. The cause of fetishism is often complex, but frequently includes doubts about the patient's own masculinity or potency and feelings of inadequacy. Treatment is normally long-term counselling and/or drug therapy
References
Paraphilia and Distress in DSM-IV (http://www.uno.edu/~asoble/pages/dsmiv.htm)
Int J Psychoanal. 1965 Jan;46:64-80. Foreskin Fetishism And Its Relation To Ego Pathology In A Male Homosexual - Khan MM.[1] (http://www.circs.org/library/khan/index.html)
The foreskin or prepuce (a technically broader term that also includes the clitoral hood, the analoguous structure in women) is a retractable double-layered fold of skin and mucous membrane that covers the glans penis and protects the urinary meatus when the penis is not erect.
In humans, the outside of the foreskin is like the skin on the shaft of the penis but the inner foreskin is a mucous membrane like the inside of the eyelid or the mouth.
The foreskin, being the part of the male anatomy removed by circumcision, is frequently used by biochemical and micro-anatomical researchers to study the structure and proteins of human skin.