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).
- Rictor Norton (Ed.). "The Shortest Way with Whores and Rogues, 1703". Homosexuality in Eighteenth-Century England: A Sourcebook. Archived from the original on 2005-12-17. http://web.archive.org/web/20051217004809/http://www.infopt.demon.co.uk/1703shor.htm. Retrieved 2005-12-31.