The Joker accurately surmised that in private, the Boy Wonder and the Dark Knight were sexually inseparable.  Though Robin was much younger, he obviously worshipped Bruce Wayne---both mind, and body.  Without much effort, one could realistically see that Bruce Wayne protected and coveted the boy sexually.  The two heroes were not only crime fighting partners… but fervent lovers.  It would be their “Achilles’ Heel”—the one weakness that could be effectively used against them.

The Joker would exploit the intensity of this relationship. He would utilize it to bring the duo down.  His plan was simple. Stage a crime, separate the two crime fighters, and abduct the Boy Wonder.  Subdue and drug the boy; then, use him to lure the Batman into an inescapable trap from which neither would emerge.  Once overpowered and secured, he would seduce the drugged boy and replace Batman as a subject of desire.  It was genius, pure genius.