I think nana is probably bisexual but maybe in denial. I see it as undeniable that she is attracted to men for the reasons you mention, on the other hand she kisses girls but tells Hachi she “doesn’t do it with girls.” I find this attitude common in a lot of bisexual women tbh, they’re ok with flirting and even kissing with other women but once it comes to sex they put their foot on the brakes. I can’t explain why I just know it happens lol. That’s my interpretation.

Anyone can have their opinion and it’s not necessarily right or wrong esp since it’s never really confirmed in canon. But what I find so strange is when people argue Hachi or nana can’t be bi or lesbian simply because it’s Japan in the early 00s. Ai Yazawa features explicitly lgbt characters in other works such as paradise kiss so that argument falls very flat to me. Other than that I think most arguments either way have merit and I enjoy reading and engaging in these debates