I seem to be in the chatter mood again... the Erl King, as you know of course, is the models/root for the Erklings who live in the Black Forest (Fantastic Beasts and Where to Find Them).
Rereading that poem, the Erl King must also have been an inspiration/model for the Goblin King of 'The Labyrinth', all connected to the fairy legend of stealing children, I suppose. Which, then, would have the sexual implications, I guess. I really know nothing, I'm just guessing and supposing wildly here. I feel like back at school, waving my hand...