Severus never stood a chance. Years of a solitary life had left him starved for both attention and touch. Once Sirius and Remus decided to coordinate clasps on the shoulder, brushes of knees, hands on arms and quick, there-and-gone-before-Severus-could-react cheek kisses, Severus' resistance was lost on a sea of need.
Ooooohhh eeemmmmmyum. Hee. This would totally be the case.