It's for the best that Levi doesn't touch Erwin's stump. He's one of the few people who could without advance warning, but now is not the time. Erwin doesn't know if he'll ever reach a point where he'll be able to view the stump as something worthy of love and affection, but that point certainly isn't now. Him touching it would also ruin the illusion, fragmentary and only in Erwin's head as it is, that he still has both arms and can hold Levi with both at once.

Erwin laughs a little when Levi pulls back and shakes his head. "You're doing too well." He leans back in, nudging Levi gently with his nose and kissing him at the corner of his mouth. "You keep grinding on me like that and this is going to be over fast."

Is Erwin enjoying being edged? Yes, maybe a little A LOT definitely. He's trying to keep things in his pants and go at Levi's pace, but it's getting progressively more challenging.