Unnatural act

Unnatural act is the term, once common in legal parlance, for certain sex acts, including anal sex, oral sex, other non-procreative sexual practices, incest, or procreative sexual acts in the wrong position or without procreative intent.

The usage of the term "unnatural" in this context doesn't 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's implied in a pejorative sense).

Unnatural acts in this sense are related to the concept of sodomy but also includes "crimes against nature" like bestiality and necrophilia.