Crimes against nature are specific sexual crimes that were considered "unnatural" in the past or are still considered so in certain places today. These acts include sodomy, bestiality, fellatio (oral sex performed on a man), and cunnilingus (oral sex performed on a woman). They were, and in some places still are, considered crimes against nature because they deviate from the traditional version of sex between a man and a woman.
The laws of sexual morality have generally relaxed as time has passed. However, in the past, serious consequences could occur for anyone who was found guilty of these crimes. Bestiality is largely still considered a serious offense. However, the other "crimes against nature" between consenting adults are much more favorably viewed by society today. The laws regarding these sexual acts have gone through many changes as society has progressed. As time goes on, it is possible that more changes will occur.