I think it's exactly this "labelling" that often does more harm than good. Feelings, heck, even food preferences or sexual orientation are not binary, "either/or" topics. They're almost always in a spectrum. There are many ways to love another, just like there are many ways you can dislike someone's company. Having some kind of "definition" for what constitutes the feeling of love only leads to the same problem you describe below:
Labels are good frameworks to simplify things. Aside from that, yes, mistaking them for absolute laws that suffer no deviation or nuances is reductive and can be harmful. I reread Ch 65 and it reminded me of my problem with this: Rika isn't confused at all. She did state clearly what they were and why they should break up. Gin is her friend, and he would be happier if they stopped pretending they were a couple.
And Gin completely misses the point by trying to frame their relationship with labels and behaviors that don't fit. He says he wants to be the closest one to Rika so that he can take care of her, but he doesn't need to be her boyfriend or husband to do that.
Shameful reference, but just like in Grey's Anatomy, he can simply be
her person. Like Cristina is Meredith's person! Even if Derek was the love of her life, Cristina is the one she trusts the most in all things, she's her best friend. And it makes sense, there are friendships that deep, going pretty much above and beyond filial and romantic love. So much so that if a love interest makes the mistake to ask them to choose between them and that friend? It's that friend that's going to be picked, the one that's been there for them for decades, through thick and thin.
If Gin really cared that much about Rika, he wouldn't have stayed around hoping that she would change her mind at some point. That was selfish of him. He got to hold her hand, go on dates, kiss and fuck her, all that helping to sell his delusion.
If he wants to 'marry her as a friend', meaning sticking around all their lives as friends, he needs to stop hogging the partner seat, and vice versa. Friends see partners come and go in their friends' lives like a carousel, but they're a constant. His childish proposal is just him bargaining and wanting to stay in denial about the true nature of their relationship, when it's clear now for Rika.
And finally, I'll say that I'm never a fan of long-winded promises and faraway deadlines. It's usually framed as romantic but it's so strange to depict people being able to be kept in stasis for years instead of getting a resolution now.