I imagine Frank is baffled by Matt not being angry at him at all, and slightly terrified when he realizes that Matt interpreted the arm-breaking (what Frank thinks of as horrific thing that he's disturbed that he had the capacity to do) as a romantic gesture of Pure Love

That, and frankly very, very scared now of accidentally hurting Matt and him not defending himself. Maybe they have a super intense kink negotiation before the sweet make-up sex? And Frank makes him promise to fight back?