For those who don't understand why Kotoka feels she needs to cut her hair:
In my opinion, a female (especially Kotoka in this context) has a stronger sense of insecurity compared to a male. This could be different to each person, but this trait does exist in my past ex and current gf.
She knows Tatsu loves her because it was all started with her hair, but when they grow old or she needs to cut her hair would Tatsu still loves her? This is one the sense of insecurity that constantly plaguing girls which is why she needs to test him a lot (leave the house, fetish ban, cut her hair) especially when they're about to get married. Marriage (by love) is a huge turning point to girls; one where they will dedicating themselves just for one person in their lives, thus if a guy trying to tie the knot with her, he needs to 110% convince her that he'll still love her no matter what because she wouldn't want the guy they married with is cheating on her.
Think of it this way: If you're in love with someone who loves your physical appearance, once you're getting old or something happened to you how you can be so sure that they're still gonna love you? Especially since the person you love keep showing strong interest to your physical appearance (Tatsu holding Kotoka's hair before holding her).
Yes, for us guys this might feel so unfair. But in my experience, that's how female mind work: the helpless constant feeling of insecurity.
Source: My current gf's feeling of insecurity, even when we're about to get married soon.
PS. For people who support Tatsu's decision to dump Kotoka and go with childhood friend instead, that's EXACTLY what Kotoka is concerned about. Life is not only about sugar, there will be times where they will fight and I think Kotoka wouldn't want Tatsu to leaves her during the hardest time.