Sirius' eyes are blurred with the alcohol he's consumed and the memories he's tried to hold back. He sways when he stands to greet Remus, who would be lying if he says he isn't a little disgusted.
Feelings aside, Sirius advances toward him with the intention of seducing him as he has before. His drunken mind allows his hands to freely pull Remus toward him by the neck and attach his lips, surprisingly firmly for as trashed as he is, to Remus' own.
It's happened before, and, although Remus knows it's strictly the alcohol, he knows it will happen again.