So, you mean something like this?
Optimus Prime shrank and concentrated until he took on the appearance of an average Hogwarts student. After being hit with that wizard's spell while battling her mortal enemy, they had both been able to transform their metal bodies into any shape, even that of a simple, smooth human. As long as no one touched him, Optimus could pass as a being of flesh, which was most useful when trying to track Megatron.
The large bathroom that had been emitting the Decepticon's signature echoed with muttered words. "...here. It has to be here, the chamber-"
Without a sound, Optimus launched himself across the room and into Megatron's side. Their grappling caused an unholy clanging to sound in the room, but this was the advantage of attacking Megatron in a wizarding loo -- the room was spelled soundproof.
"Wait! Wait!" Megatron screamed even as he punched his fist into Optimus' "kidneys". Optimus transformed into a larger human than Megatron could emulate, and he quickly overpowered his ancient enemy, pinning his wrists to the cold, hard floor and sitting on his "groin".
Megatron bucked and moaned against Optimus' weight, but he could not dislodge the Autobot before they both felt the very foreign sensation of a new accessory growing between them. Megatron groaned again, this time with an abandon he didn't realise his human form could muster.
Optimus leaned over his foe and dragged his "lips" along Megatron's skin. The scraping sound would have flailed true human nerves raw, but Optimus found the rough grind enticing and Megatron's writhe in response even more exciting. "I like this better, too," he whispered as a new part of his own made itself apparent to the robots' awakening.