Is 'sorbet sex' a well recognized expression?

Speaking from a North American, big city perspective, I don't recognize it. I also don't know of a specific word or phrase to describe this type of relationship. I think the Guardian author was inventing a new phrase -- that's why he placed it in single quotes. If you want to use it, I think you would have to give an in-line explanation as the Guardian author does in your original post. Otherwise, you might just say "casual sex between relationships", "no-strings-attached sex," or some such.

Edit per comment: sorry, was just trying not to spam.

I had a look through the Google Books results that @Kris posted in the comments and every single instance of the phrase (in some cases, the result seemed to be in error) was accompanied by an explanatory sentence or a full blown definition. In many cases (here, for example) it was considered so obscure that a Glossary entry was included to explain the term.

Combined with the fact that many of us don't seem to have heard the term before I feel safe saying that no, it is not a well recognized term.