We get the term "crimes against nature" from 12th and 13th century theology and logic (I'm so a history major).

Basically, the European thought at the time theorized that all things were in a state of ascension. Jesus had been born a mortal, transcended that and ascended to divinity; people worked through life to ascend in the afterlife to heaven; and so, in a corresponding, perfectly symmetrical world, was everything else (a nut became a tree, became lumber, became furniture, became firewood, became smoke and ascended to heaven, for instance). All things and all actions were working in accord with the divine precedent, godly will, nature -- what have you -- to create something better.

Except, of course, that some things didn't. Religion came in handy to explain this gap in logic, however -- those things that weren't creating something or moving towards perfection were evil, against the course of nature: unnatural.

Any form of sex other than strict penile-vaginal intercourse fell squarely into this category. Oral, anal, masturbatory or homosexual fun produced nothing, was a biological 'dead end' -- and so was against nature, and being against God, was a crime.

This sort of thinking is long past outdated in scientific thinking, but it served as a basis for several now-fundamental rulings (mostly in response to the Reformation) and thus has stayed with us, leading otherwise perfectly sane people to call gays "unnatural" with a straight face. (Okay, that's a bit of personal opinion there, but whether you agree or disagree with homosexuality I have yet to find anyone who can justify it as unnatural).