Updated "atfsummary" (Anatomy & Trauma summary + aesthetic anus fetish paragraph):
The human anus when closed at rest should resemble an anteroposterior slit surrounded by shallow, semi-symmetrical, radial perianal skin folds. Healthy perianal skin commonly has a different, typically darker, color due to physiologic hyperpigmentation.
Anorectal trauma can result in persistent, externally-visible changes to the anus and its surroundings. Potential consequences include the anal orifice shrinking or becoming misshapen due to constricting scar tissue formation, the perianal skin folds disappearing due to underlying anal sphincter damage, and one or more lumps appearing (e.g. an anal skin tag, prolapsed internal hemorrhoid, or thrombosed/diseased external hemorrhoid). If the perianal skin folds remain, pruritus and/or cutaneous thickening may result in exaggerated skin folds. [References for paragraphs 1 and 2: Anatomy & Trauma]
A [strong] sexual fetish must be satisfied for sexual arousal, and a fetishist with partialism obsesses usually over a non-sexual body part. A human anus resembling the anatomical ideal at rest has symmetrical features; symmetry is one aspect that humans consider when evaluating beauty. Partly due to common prejudices against and denigration of the anus along with rampant, anus-mutilating erotic anorectal violence, developing an aesthetic anus fetish is highly unusual; and _accepting_ it, even moreso. This fetish does indeed seem to be very rare: otherwise that violence would have strong opposition. Some malicious people (such as sexual sadists and sadistic psychopaths) pretend to have such a fetish—and will even praise a clearly damaged/diseased anus typically without being challenged—because doing so facilitates and perpetuates anorectal violence.
1. "The DSM diagnostic criteria for fetishism." Archives of Sexual Behavior. 2010 Apr; 39(2): 357-62. PMID 19795202. doi:10.1007/s10508-009-9558-7.
2. "Symmetry, beauty and evolution." Nature. 1994 Nov 10; 372(6502): 169-72. PMID 7969448. doi:10.1038/372169a0.