I do agree with you, Ren just needs to lose some of the icy chill he got goin on when he says it.
Though I dont blame him since he loves her, but when dealing with this chick you gotta be as straightforward as possible cause she's a special level of dense.
Honestly, though, I would have been happy if Ren said- I am angry that you got kissed by Fuwa and let your guard down.
Its so sneaky of him to manipulate her like that and not confess himself first
Especially Ren but both of them need to learn to be honest with each other.
Honestly to me. Ren is just stating his opinion though it seems harsh what he said isnt wrong. I'm glad Ren said this, it may not be the best way but it'll help drive the romance plot forward.