I don't think any of these are what your are looking for but they do have Harry/Katie in them, they are mostly smut although Girls, Gils, Girls does have a lottle plot:
Girls, Girls, Girls by Scorpio at AFF - http://hp.adult-fanfiction.org/story.php?no...2&chapter=1
'Quidditch Practice' by Diviniti at AFF - http://hp.adult-fanfiction.org/story.php?no=600014488
A Perfect Day for Flying by pettybureaucrat at Ficwad