He's not surprised at first but rough? Rough is not normally in the other's anything and Jim loves it. He can't help but be pliant against him, a whine slipping out and lost between them. There's a faint consideration in the back of his mind that if Sherr stops now, he's going to stab him - gently and nonlethally, of course.
And he's far too distracted to notice he's loosing length on his tie. Jim's too busy gripping at Sherr's shirt like it's a lifeline.