I really hate how people feel the need to argue that a character is not really bi and is actually a "pure" lesbian. As if there is something wrong or dirty about someone being bi.
Isn’t this the opposite case though? Miki has 100% dated men before, and likely had sex with them as well, but what people are saying is that because she was actually in love with Kana the entire time there’s no real evidence for her being bisexual. She specifically states “I tried my best to make it work with other people but...” which implies that the main reason she chose to date them was in order to force herself to get over Kana. That smacks of classic compulsory heterosexuality to me, just like above comments have also mentioned. Some people only realize they are lesbians later in life, and some of those people have already married men and had kids when they realize. That’s the vibe I got from this.
As far as sexuality goes all we know is that Miki loved and loves Kana. Maybe we should just take a page from Handsome Girl and Sheltered Girl and call her Kanasexual lol