Hey! If you're going to cry... let me watch!
Fujimoto is a comedy genius. If you don't appreciate how fucking great that line is it's on you. Peak tragicomic chapter, really funny, really sad. Also as foreshadowed like seventy times Yoru can't or doesn't want to turn Denji into a weapon and she found a way to better her bluff of not turning Denji. I think its pretty clear she is not manipulating Denji, she is not in control, she's being a tsundere because she doesn't want to admit otherwise. That's why she literally tells everyone the opposite. Now I really wanna see Asa's reaction after all that heavy petting.
EDIT: I have a theory, it's possible that Yoru, knowing that for some reason she can't turn Denji into a weapon, made this pact in order to be able to turn him given the right circumstances. But maybe I'm overthinking it.