I think that in an old discussion about this, someone convinced me that it referred to the relationship for the entire summer. 
I didn't think he was referring to FNIT; my sense was that he meant the summer "fling." Actually, on my first viewing, when I didn't know yet what was to come, I still didn't think he was referring to just FNIT. Why I think this is because he didn't say, "That
was a one-shot thing..." He said, "It's a one-shot thing we got going here." Present tense, not past. Which made me think it was going to be an ongoing thing, or Ennis was willing for it to be, something like that. He sure didn't say, I ain't queer and you lay a hand on me and I'll kill you.
kathy