Clinton's statement hinges on your definition of "sexual relations". For many, that means "any kind of sexual activity". I grew up in the South and there I understood it to mean good old p-in-v intercourse. If you got to third base with someone (oral) and a friend asked you if you had "sexual relations", you would honestly answer "no".

I believe this strict definition comes from being a highly Christian culture where certain sex acts are proscribed and others aren't. "Sexual relations" is really defined as "the sex stuff the Bible says you can't do outside of marriage."

Of course, at a high level, Clinton was very clearly trying to obscure the facts of his relationship with Lewinsky, but whether or not that sentence was literally a lie depends on the definition of the words in the sentence and there is a reasonable interpretation (Clinton himself is from the South) under which it is factually true.