I've watched this, and I think his horniness levels is valid. He was a wizard(30yo+ virgin) who died regretting it. Then in a newborn's body.
His new parents are younger than he was, and has difficulty seeing these strangers as family (for now?). How would you feel about suddenly breastfeeding off a hot young stranger?
He also mentions how he doesn't actually feel turned on "perhaps because she's my mother?" but more likely because he's a toddler.

I think Twitter is just blowing their top over this because it shows a happy cis couple boning.