http://en.wikipedia.org/wiki/Crime_against_natureA crime against nature or unnatural act is a form of sexual behavior that is not considered natural and is seen as a punishable offense by law. Sexual practices that have historically been considered by some to be "crimes" against nature include anal sex, bestiality, incest, miscegenation and necrophilia. Other examples include heterosexual and homosexual fellatio and cunnilingus, other non-procreative sexual practices, or procreative sexual acts in the wrong position or without procreative intent, even between married couples.[1]
"Crime against nature" is a legal term used in published cases in the United States since 1814.[2] The term is also used in dozens of countries and several U.S. states. It is sometimes also used as a synonym for sodomy or buggery.[3]
The usage of the word "unnatural" in the context of "unnatural acts" does not carry the same meaning as "the opposite of natural", and says nothing about whether an act actually is or is not a product of nature (though that may be what is implied in a pejorative sense).