Wow. What a brilliant idea. Your Ivo is a great original character, and I loved the outsider POV. To see Minerva and Severus (or rather the Professors Snape and McGonagall) through the eyes of someone who doesn't really know them makes their appearance all the more impressive. I love how fierce you show them to be.
He had not seen them naked before. They looked ghostly, ethereal, like something of another world: Snape with his narrow torso and stiff, pale cock; McGonagall with her black hair in a wild cloud about her shoulders, her breasts gleaming silver-white in the moonlight.
Hers wasn't a young woman's body, but it flooded Ivo's mind with images of Valkyries, Amazons, all the valiant, bare-breasted warrior women of the centuries. He gazed and felt his own cock rise, and he wanted to groan aloud, as Snape did, at the sight of her.
A stunning image, and so hot! And at the same time a great piece of characterization.
The world had re-opened for Severus Snape and Minerva McGonagall.
Ivo felt, obscurely, as if something should happen to note their passage: thunder should sound or lightning should flash or at the very least, magic should shimmer round them, perhaps leave a scorch on the grass to show where they'd been.
But of course nothing of the sort occurred.
Snape and McGonagall simply stood quietly for a moment, unremarkable and unremarked. Then hand-in-hand they stepped forward, and through the gate took their solitary way.