My own personal fave 'established relationship' to write is actually not even a sexual one. It's a familial and friendship one. It's also the one I've been writing the longest. In fact, the very first story I actually publicly posted was this pairing -- Methos and LaCroix. I love writing these two! Two thousand odd years of history, starting with LaCroix when he was still mortal, a twelve year old boy, and his aunt's new husband, Methos. Their relationship is the foundation of all the relationships in the series I write. Though when I wrote that first story eleven years ago, I had no idea that Methos and LaCroix would still be such a big part of my fandom life! They've outlasted just about everyone else I might have fancied over the years -- I guess I have an established relationship with them! [g] And I still love writing them. They snark so nicely at each other. But beyond the snark, there's an understanding and an acceptance of each others natures that I never seem to tire of exploring.
What established relationships do you enjoy writing?