It's all in our history I suppose. The term stems from the Book of Genesis (chapters 18-20) and tells of how God wished to destroy the sinful cities of Sodom and Gomorrah. Since then adopted into Jewish, Islamic and some Christian law as well. In a way just a leftover - some might say ignorant - law still persistent with some legislations.
The term really has little modern usage outside Africa, the Islamic countries and the United States. In the various criminal codes of the U.S. the term "sodomy" has generally been replaced by "Deviant sexual intercourse", which is described as any form of penetrative intercourse or cunnilingus between unmarried persons.
The laws have often been challenged and have sometimes been found unconstitutional or been replaced with different legislation. Elsewhere, the legal use of the term "sodomy" is restricted to rape cases where anal penetration has taken place.
So call it history, or upbringing, or society... But usually when people find out there's nothing deviant about anal sex, that it is in fact just as much as pleasurable act as all other sex acts, the walls of 'dislike' are quickly followed by those of intense 'like'. Personally I wasn't much different. My ass was off limits, period! That was, until I met a girl that showed me a thing or two