Whenever Joker goes out under cover (as a regular civilian) he wears a scarf to cover his mouth and hide his teeth.
Now imagine he meets Bruce Wayne, they hit it off and start dating, both completely ignorant to who the other really is.
Joker goes out more as a regular person now that he has a boyfriend. During the day they date, and by night they fight, both careful not to reveal who they really are.
Bruce has seen the man without his scarf but it’s extremely rare and he speaks smaller, facing down to hide his mouth. Bruce just thinks he’s shy, and finds it adorable.
During an intimate moment together, Jokers scarf removed so they could share kisses, Bruce finally tells his boyfriend his biggest secret.. he’s Batman.
The Joker laughs like it was a bad joke, laughs so hard that Bruce sees his sharp pointed teeth and instantly recognizes the smile.
Joker slaps his hands over his mouth, realizing what Bruce had seen “… You’re not serious” he asks
And suddenly they realize who they have been seeing..