FOR NOW
Naruto and Sai don't really have a relationship, per se. They don't have a relationship because Naruto is still in love with Sasuke and Sai can barely comprehend friendship, let alone limerence.
They don't have a relationship; they just have sex sometimes in the hazy summer afternoons at Sai's apartment because Naruto's has too much of Sasuke in it still, and Sai wants to be sure that it's him whom Naruto's fucking, not some other dark-eyed ghost.
Sai has never taken back what he said about Sasuke that day, because as far as he's concerned it's still true (he also has opinions about the size of Uchiha Sasuke's penis, but that is beside the point). He hasn't repeated himself since that day; he is content to wait for Sasuke himself to prove him right.
They don't have a relationship: they're on the same team and watch each others' backs and sometimes Naruto slams Sai against a wall or pins him to a tree and shoves his tongue in his mouth.
They don't have a relationship; they just do it because it works for now. And sometimes, in the morning or afternoon or evening, or at night when there's no mission and they're pressed up against each other like chidren and all they can hear is the throb of another human heartbeat - they can believe that they might be able to make it work forever.
FINIS
Word Count: 184