I wouldn't have said it's progressive at all. In fact it's borderline homophobic. The episode uses Pikachu and Piplups brief infatuation to generate humour implying that it's somehow funny, weird or unusual for a boy to be attracted to, display affection for or love another boy. This is compounded by the fact it was involuntary infatuation created by a machine. That, unconsciously perhaps, reinforces notions of Heterosexism, suggesting that the only valid forms of attraction and desire are between people (or Pokémon) of the opposite sex and that everyone else should be treated as a joke.
Sounds very SJW-ish to me. I didn't think there was a big issue with how Pikachu and Piplup's attraction was set up.