It is extremely difficult not to react to a statement like that, but Arashi manages to keep it to an abrupt, violent tension in his whole body, and bites his tongue because he did promise that he would stay quiet. He doesn't actually know what to say to that, or rather what he should say. Never once has it even occurred to him to hit Harvestman (although there's been plenty of times where he's wanted to grab him and shake him).

Unfortunately, he knows that it's true; it hasn't been difficult to gather up to this point how Harvestman handles his personal relationships, as well as the conflicts within them. All he can hope is that Harvestman knows he would never do that.