Kun is mostly used for males. It is also used by grown-ups for kids, someone with a lower status than you or if a girl wants to flirt with a boy.
Or a male friend, but if you're close friends you stay away from suffix. Chan is an exception.
This doesn't count in every manga ofc, but in Kishi's mind I think this is what it means. But remember, in OP Sanji uses San with Nami and Chan with Robin. Still he loves them both equally. :3
Oh alright then , now that i think of it I never saw a girl call with the Kun suffix so your right
On an other note , I dont think that the canon couples use suffix for there lovers , Minato and Kushina didin't use one , same for Asuma and Kurenai and I dont think Dan and Tsunade use any of it too. So i guess the fact that Sakura didin't use suffix for Naruto is not that bad after all.
Edited by Kakashi-Sensei, 18 July 2013 - 05:32 PM.