First of all: people in romantic relationships are allowed to have friends. That's normal and healthy. You'll probably have a more successful relationship if you don't have to rely on your partner for 100% of all social support.
Second: Having people look out for you and getting help from people isn't a bad thing. There's this thing called society, and it's actually really great. Sharing responsibilities, allowing people to specialize, do things other than spend 90% of their time gathering food, distributing the load of hardships. As the saying goes, "standing on your own two feet doesn't mean you have to stand alone." Why is no one telling Miwa it's okay to rely on people?
Third: Why aren't people calling out Miho for being awful?
Fourth: People can potentially change. Usually, they don't even mean to, but sometimes when the put in effort, they can change some things about themselves. Since Kan is being obtuse about the change he means, it's hard to say just how difficult the change he's talking about is. But even if he's talking about sexual orientation, there's evidence that a small percentage of people do involuntarily change over time.