Yuri manga like this would have a lot more credence if they were at least /set/ in an earlier, less tolerant period.
Doea there rly need to be this much heteronormative self hate in a modern Tokyo setting where, last time I checked, gay marriage is legal?
Gay marriage is not legally recognized in Japan. Some cities, including districts within Tokyo, will issue partnership certificates, not necessarily recognized by other such districts, and which don't convey any legal rights or protection (but a hospital or landlord might choose to honor one). Polls show some broad support for acceptance of gays, but as of 2019, the ruling LDP still had seated members calling gays "unproductive". National law still doesn't ban employment or housing discrimination based on sexuality.
I see nothing implausible about someone's background or family being steeped in heteronormativity, still.
And the new girl is living at home on some rural island, just the sort of setting to be more conservative. Even if her parents don't care she may not know that, and doesn't want to risk finding out.
Wikipedia says
"Same-sex couples are not allowed to legally adopt in Japan. Lesbian couples and single women are unable to access IVF and artificial insemination.[26]
In April 2017, Osaka officially recognised a same-sex couple as foster parents, making it the first such case in Japan."