Not sex, really, but words that refer to sexual acts.
This Slate article, by the editor-at-large of the Oxford English Dictionary, sheds light on the difficulty of defining sexual words that some people would rather not see in the dictionary. And not just the words, but phrases that use those words in proper context.
Don’t read the article if you can’t handle the language. The author says, for example:
I will die with a certain measure of satisfaction knowing that I put the sentence “Butch fucked the mannikin through the hole he drilled in its crotch” into the Oxford English Dictionary.
There are important elements in that example. The mannikin is not animate. The hole is not a vagina. Most dictionaries list the sexual definition of “fuck” as being sexual intercourse. Then they define sexual intercourse as being something that a man does with a woman (or at least that two people do with each other). But most English speakers would consider the mannikin example legit.