It’s okay for bi girls to headcanon all of their favorite characters as bi. It’s okay for pan girls to headcanon all of their favorite characters as pan. It’s okay for lesbians to headcanon all of their favorite characters as lesbian. There is nothing wrong with that. If canon doesn’t specify a character’s sexuality, and instead codes them as default heterosexual, a bi girl headcanoning said character as bi or a lesbian headcanoning said character as lesbian is perfect and awesome.
If a bi girl sees a female character who has canon romantic relationships with men but has heavy romantic/sexual feelings implied for another female character, it’s okay for her to headcanon her as bi. If a lesbian sees a female character prioritize women, despite being in canon het relationships, she can headcanon her as lesbian who struggles with compulsory heterosexuality. Both forms of headcanoning are great and A+ and not problematic or biphobic/lesbophobic.
The problem only emerges when you encroach on canon-specified sexualities. If a female character explicitly states or is explicitly specified as bisexual, don’t call her lesbian or erase her bisexuality. If a female character explicitly states or is explicitly specified as lesbian, don’t call her bisexual and don’t ship her with male characters. We have very limited canon content, so we should respect that and celebrate the canon that we have without stepping on each other’s toes or spewing hostilities at each other.
Also, here’s just another disclaimer: a lesbian character getting killed off while her bisexual/pansexual girlfriend is traumatized and/or paired off with a man is not “good” or “radical” representation for lesbians or for bisexual women. It just reinforces the “dead/unhappy lesbian” and “traumatized/always-ends-up-with-a-man bisexual” narratives and that’s really, really bad for all wlw.
All wlw headcanons are great and awesome and amazing and perfect, and there is no one headcanon that’s “better” or more “radical” than the other! If you’re a wlw who sees herself in a female character that’s coded as default heterosexual, you have every right to headcanon her with your specific sexuality and no one should fault you for it!