What Does "68 I Owe You One" Or "68 You Owe Me One" Mean?

It means when a guy says ugh..well here's and example

girl: How about a 69
guy:how bout a 68
girl: Whats a 68?
Guy: You do me and ill owe you one

so its wen a girl gives a guy head and she's thinking
shes getting something in return but the guys says 68 and ill owe you one
which means haha the guy ain't commin back to give you one.

I believe thats a sexual connotation