Joder. As simple as that, The sexual act, and with all its figurative implications, exactly as in English. But the verb "joder" has been left by the use as an "out of fashion" term, a dirty word, but actualy it doesn't keeps the primary meaning. Therefore, if you want to call the sexual act by its name, the real one, each Spanish speaking country has a couple of ways to call it. Making a list of them, is not the scope of this site.

in mexico use "chingar" this word can be use in many ways as fuck its a hard bad word

http://www.thefreedictionary.com/fuck

The word´s meaning depends on how you use it, much like you would use it in english. It either can be an insult or an emphazised exclamation in bad terms (in this case it can mean a lot of words which are not even related to the sexual meaning of the word), and also can be used to refer to the sexual act itself in a very rude way.