That's true, especially given the crushing on that person primarily as the opposite sex. I could definitely see our straight boy lead being straight but still attracted to his girlfriend even when she's a boy because, y'know, she's his girlfriend. And/or the romantic attraction => sexual attraction pipeline. In that sorta situation it's definitely up to the person if they wanna identify as bi, heteroflexible, straight with an exception, or whatever. But I do think all the anxiety about it is likely to get resolved by accepting he can like boys without being weird or less of a man.
Also our girl is bi, there's zero doubt about that.
I do wonder if the transforming characters have any genderfluidity or if they're cis and just accept physically transforming sometimes.
Now kick my post through the wringer because of all my English terminology that I dunno how well it maps onto Japanese.