2 chapters out of 7 volumes
Are you seriously arguing about something you haven't read
Someone who's read 7 volumes just made a statement, and you're going to argue that, without knowing anything of the circumstances and reasoning of the character? Are you absolutely fuckin serious
So she didnt date a dude and was going to happily marry him if he didnt dump her? She is open to herself about dating guys and girls. How is she not bisexual?
Whether or not she'd truly be happy with being in a relationship with a male, or if she'd truly love him, is still unconfirmed (speaking as someone who has only read the last 2 chapters). She mentions in chapter 1 that her goal from the get-go was to live an "ordinary" life as a woman (get a fulfilling career, marry a man, have kids, have grandkids, etc.), and after Yuuji asks if Nanami is a lesbian, there is a hint that she might be one (possibly from an incident in the past). In chapter 1, she is pretty fixated on the idea of living that ideal "ordinary" life, but Kae-chan indirectly and implicitly shows her that it's okay to stray from that path (and love girls, ayyy).
Furthermore, in chapter 1, Nanami reveals that she didn't visit her hometown because she didn't want to see Yuuji face-to-face. That's not exactly ideal, especially if they were planning to get married - so no, they wouldn't have been "happily" married to each other in the long run (or even in the short term, for that matter).
We also see throughout these two chapters that Nanami and the people around her are fixated on superficiality. Nanami wants to be the perfect, ideal woman (again, with a fulfilling career, happily married, stable family, all that jazz) on the surface, but her plans crumble when Yuuji steps out of her life and Kaede walks right into it. Her wanting a "boyfriend or girlfriend" is likely related to the fact that she still hasn't gotten over wanting to have the perfect lifestyle, but we are given no evidence that Nanami is actually into men. She just wants (or wanted) one to make herself look better.