Why are sexual fetishes studied as Abnormal Psychology?
Abnormal psychology is the field of psychology that studies unusual patterns of behavior, emotion and thought, which may or may not be considered as a mental disorder.
Sexual fetishism is a sexual focus on objects or nongenital body parts. Sexual fetishes are used as non-pathological aid to achieve sexual excitement. But there are just certain fetishes that are just way beyond normal. It's always a pre-existing mental disorder that usually causes a weird fetish.
Sexual fetishes doesn't fall under the usual pattern of behavior, emotion or thought during sex. But it doesn't mean it's a bad thing. Typically, the usual thoughts, with it comes to sexual activities, usually involves a penis, a vagina and a pair of breasts.
There will always come a time in a couples' life, when the usual becomes boring, That's why they got to try new things every once in a while. Well, that includes trying unusual sexual positions("Kama Sutra"), sexual role-playing(Police-Criminal or Master-Slave) and sex toys.
Sexual fetishes are just unusual that's why, it falls under abnormal psychology. But it doesn't mean it's not acceptable to do it.
A biological/evolutionary perspective is also interesting.
Biologist would define any animal behaviour that gets in the way of survival, reproduction and passing your genes on to the next generation as being abnormal and maladaptive in a longer-term evolutionary sense.
Its interesting too that a number of strange sexual behaviours have also been observed in other mammals, aside from humans. This suggests that human sexual behaviours are not all psychological, but may have some deeper biological roots.