They do not do this when the moon is full, when it is his time of the month. Not with the biting and scratching; not with blood and saliva and less savoury fluids flowing freely. It is simply too dangerous.
They do it in the dark, when there is no moon, and in the half-light of wax and wane. It does not matter that they each come - in both meanings of the word - in human form, for they know what they are inside: They are animals; Sirius by choice and Remus by force.
The black dog is an omen of death indeed; a little death suffered a thousand times yet never enough. Remus mounts his lover's back and growls into his ear, "Fuck me," and Sirius twists to pin him to the ground.
There is sweat in the hollow of Remus' throat, and Sirius dips his head to lap at it - to drink of it - to taste the submission of the defeated wolf. The hands of each are in the other's hair and knees are forced apart, and when Sirius dives in to pin Remus on his cock, Remus' own jabs sharply against his belly.
The thrusts of the top dog are swift and harsh; careless in their urgency. There is no love here; only the rutting of wild beasts: Fuck and be fucked - be violated - be dominated. When Remus throws his head back and howls, his throat is bared to the silver sliver that falls across them, and Sirius latches onto it once more. The wolf wears his bruises like the collar of a dog, and the marks of Sirius' teeth scream to whom he belongs.
Tomorrow, it will not be like this. Tomorrow, Remus will overtake Sirius while the latter is still on all fours and bite down through the matted locks at the nape of his neck. The wounds may be hidden, but the Animagus - and the animal - know they are there. They may be domesticated -
But they will never be tamed.
Author's Notes: Written for Porn Battle Round Three @ IJ. The prompt was Harry Potter - Sirius Black/Remus Lupin, "Fuck me like an animal.".