So maybe this is what got him expelled from Hogwarts: Leda convinced teenage!Newt to help her woo a unicorn late at night in the forbidden forest - she said she wanted some of its hair for a spell, or something. And he was madly besotted with her, so he said yes. Only she didn't tell him all the truth - because it's actually unicorn semen she needed. And they will only approach virgins - which 15 year old Leda isn't, but 15 year old Newt emphatically is. So she has him tied bare-assed to the back of this old mossy statue of a unicorn which was designed for this purpose by some nefarious ancient wizards, with his back arched and his thighs splayed, and he's really starting to regret his life choices as the fire whiskey wears off, but she promised it was just to summon the unicorn and obviously she wasn't ACTUALLY going to let it fuck him. Only she's trailing her fingers over his goosefleshed thighs and muttering about how pretty he is, and how she should have brought a camera, and then she does some kind of lubrication spell and suddenly his asshole is all slippery, and then he hears hoofbeats...

Points if more than one unicorn shows up to fuck him - maybe a foal and a stallion.
Points if it was Newt who told Leda about the unicorns in the first place, thinking she'd just be as delighted by their existence as he was, and he's already connected with them in a nonsexy way, and he's begging the foal not to do it.
Points if Leda gets off watching it, and DOES take photos.
Points if it's Theseus who discovers his little brother getting fucked by a unicorn and saves the day.