@8bit_Tengu she was cute but seeing it from his eyes, he fell in love and spent a literal decade with her in the current form and she obviously was just changing herself because of social pressure and anxiety. So to him the 'her' that he's comfortable with and always knew is the one he's attracted to (love can wire your physical attraction to someone's form) is the one she's always been. I'm sure if she wanted to change on her own volition he'd support her but I think he's just used to her and doesn't need her to change her entire body for him. In fact it might be like seeing a different person almost, like "this isn't my wife, this isn't the person I'm attracted to" on a subconscious level.
"There's no need to force yourself to change." he said - - you fall in love with an immortalish / non human being, you take the whole thing as a package deal. Love is not forcing someone to change, you bought it you keep it. You could see her true feelings in her response to when he said that.