The Expanding Man, by undunoops. Snape/Lupin, set post-OotP. NC-17 (but elegantly so).
"He fucked me in a weak moment!" Severus snarled.
"And the next time?" Remus’ mouth quirked involuntarily; Severus did so like to rewrite history.
"Well," Snape muttered, studying his fingernails, "I had no further to fall, did I?"
And he calls Sirius a bastard! "You’re an opportunist," Remus said.
"Quite." Snape agreed.
Very funny. Wonderful characterizations. And, yes, mpreg.
(Terrific author AND artist. Lucky so-and-so!)