Nope, their relationship just doesn’t make sense. Its on when its convenient to be on.
It’s in order. In 5 and 6 it seems that Fry and Leela are doing it, sometimes maybe, and Leela is ambivalent about it and Fry is into her as always. There’s no rhyme or reason, just when it’s convenient.
Well that is frustrating and disappointing! Makes me wish they didn’t end the last movie the way they did.