Disclaimer: Naruto isn't mine
Being a dad means telling her she looks better with thin hair.
Iruka didn't care much about his long hair, he wasn't Neji after all, who at times considered the well-maintenance of his hair more important than destiny. Iruka had long hair because he always had it and he got used to taking care of it.
As little as the effort that Iruka put into it, he did have beautiful thick hair that Sakura proclaimed jealously over. Kakashi loved running his fingers through it even if Iruka complained about the tangles and pain of brushing them out.
Imagine Kakashi walking in to find Iruka, Iruka, crying over how thin his hair was. "It's never been like this!"
Kakashi knelled down on the bathroom floor to comfort his husband. "Iruka, baby, your hair looks great either way. I love you for you, not because you have thick, gorgeous, brown hair. It will go back to normal at the end of this." Kakashi put a lock of Iruka's hair behind Iruka's ear.
"I miss my hair."