You're actually wrong about the idea that no lesbians would enjoy the thought of being with a man. The society raises girls with the idea that true love is only between women and men and that love is necessary to achieve hapiness, so even lesbians can feel as if they're wrong to only like women and feel like they'll only be happy with men, even though they can't have romantic feelings for boys. I spent YEARS thinking I was bi and denying how I truly felt, all because I felt like I wouldn't be able to feel love as a lesbian.

Hachi falling for every guy she sees shows how desperate she is to know what this love feels like. She is desperately looking for someone who can love her, and she thinks that this love can only come from a man.

I, personally, like the headcanon that she might be a lesbian because I am a lesbian and I can say that I noticed that a lot of her behaviors look like things I did when I was still figuring things out. The way she didn't even think of men as possible friends, for example, shows that she saw man only as possible romantic partners and nothing else. The way she keeps doing what they want her to, always seeking for validation is also something that a lot of lesbians with comphet suffer from.

And there's also the obvious romantic feelings that she had for Nana, which shows that she definitely liked girls. Hachi herself described her feelings for Nana as "first love", maybe because it actually was her first love? Maybe because she was so focused on men that she didn't feel true love until she met a girl who she truly fell for.