Enough
The first time Sirius called him 'James' during sex, Remus pretended not to hear. In the heat of passion, sometimes people said things they didn't mean. He was young and in love, willing to believe he could, for once, triumph over James Potter, the Boy Who Had Everything. He believed love was enough to conquer all.
The second time Sirius called him 'James' during sex, Remus was resigned to the fact that he'd never have Sirius's heart the way James did, but sharing his bed was enough. Remus had no one else; Sirius's careless affection would have to suffice.
Fifteen years and endless sorrows later, Remus refused to play second fiddle to a dead man, and pretended not to notice Sirius's attempts at a more intimate reunion.
But when he found Sirius cornering Harry in a darkened corridor at Grimmauld Place, murmuring, "James, please," and stroking the pale, scared boy's disheveled hair, Remus realized some things were more important than pride.
He didn't even have to think, didn't hesitate. With a flick of his wand and a murmured, "Obliviate," he ventured down the hall and led Sirius away from the puzzled Harry.
That night, he let Sirius into his bed again, and hoped he could hold Sirius's attention until Harry returned to school. It would have to be enough.