This is such a great, moving story! Your OCs and Ivo's tale drew me right in, and I was especially impressed with your handling of the whole camp situation -- the bleakness and the desperate attempt to preserve some humanity. In that sense, your handling of the sex worked on more than one levels. It's not just hot; it's more than that. Seen through Ivo's eyes, it shows us how people try to stay alive and human in an inhumane environment -- a sort of waiting loop between the death you've escaped and a new life that you hope to begin, whenever that will be. And that goes for Severus and Minerva as well as for Ivo: he's invaded their privacy, watches, and tells it like it is, but he doesn't rob them of their dignity.
Love your characters, both the original and the canonical ones, and just to quote a few of the many great lines:
--"You might allow us at least a day, Severus," she said tartly, "before you set about alienating all and sundry." -They weren't "intimate," except in the sense that the whole damned camp was "intimate," everyone living cheek by jowl, no solitude, precious little dignity. --But so restrained were the professors' movements that Ivo could easily choose not to know, and mostly that's what he did choose, in the spirit of polite obliviousness that usually prevailed at the camp. --the entire, poignant exchange in the solstice night, plus the aftermath.