I'd say D'Argo and Chiana from Farscape (mainly because someone's already said John and Aeryn).

So during a standard body-swap episode they do the obvious thing everyone would do in a body of the opposite gender, and decide they quite like each other. They have a relationship (mostly based on sex) which doesn't get a lot of attention, until Chiana decides to sleep with D'Argo's son (for unknown reasons).

Takes them probably over a season to start getting back together (I can't even recall when it happened), but in the end Chiana is pretty distraught at D'Argo's death.

Just seemed like a good "background" relationship to me.