I never actually believed that Joker really "loves" her in some way. I rather thinks that he only uses her, because he knows how insane in love she is with him. So I dont see why he should not cheat her.For Joker to have a daughter harley would have been pregnant or he would have to have cheated on her. He wouldn't do that... I think.
But I can be wrong, I dont know in there is something official regarding this.