@bellac11 You need to consider that saying "not all straight people are homophobes" IS EXACTLY THE SAME as saying "not all men" and "all lives matter".
And then reflect that those are basically neo-Nazi slogans at this point.
@crumpet On the off-chance that your question is sincere and not sneery, I'll answer. For those who have exclusively heterosexual relationships, the whole of mainstream society and history provides a welcoming culture and community. For those who have exclusively same-sex relationships, there's an (on the whole) welcoming and supportive LBTQ+/Gay/Queer culture where they can find community. For bisexual people, as they move through life, there tends to be less unequivocal welcoming; they can encounter slight suspicion in both communities as not completely belonging. The author is reflecting on her experience in both places in the article. I think the sneery tone is projected and/or from straight people who think "she's just a bisexual, not a proper lesbian, so I can undermine her account of her lived experience without being called homophobic" - maybe?