I don’t think the comment “bisexuals have straight privilege unless they’re in lesbian relationships” makes any more sense than saying “lesbians have straight privilege while they’re single”. This makes the assumption that all bisexuals who are single or in opposite-sex relationships actively hide their sexual orientation.

If a gay woman keeps her sexual identity secret while she’s single in order to avoid discrimination, we don’t accuse her of co-opting straight privilege – we sympathize with her for feeling the need to closet herself. So why the double standard for bisexuals?

It might not apply to you, but that doesn’t mean there aren’t straight-looking femme lesbians, or androgynous-looking, rainbow-wearing, alternative-haircut-having bisexuals. My point is that that comment oversimplifies and overgeneralizes things in a way that seems unreasonable to me.