It's like being mad at an earthworm for not having a backbone, it's completely pointless. Neither of them will ever grow one, they physically can't (one of them is a spineless dirt-dweller and the other is in the order Opisthopora); why bother getting mad? Can you imagine if he just suddenly asserted himself properly? He'd walk away from her and cut all communication; the story would end right there. He is narratively forced to be spineless, like the earthworm is forced by nature to be spineless. Just think of it as the story of Mizushima flirting with an earthworm named Souta and you'll laugh it off like I do.