I love eliade's fic, 'Strains May Float' where they're both happily seeing other women (and in Jim's case, men) but still fucking each other and gradually falling deeper in love without quite realising it's happening, because it's such a refreshing change to all the fics where they're insanely jealous/possessive/only meant to be with each other.

Not that I don't like those kinds of fics, too :-)