Because people don't read fantasy to have their real life notions challenged, and there's still a widespread sense that lesbians should keep that part of themselves behind closed doors. I think that the lesbian characters must step on the personal taboos of those who complain -- these are the people who think nothing of seeing coworkers with pictures of heterosexual relationships (wedding photos, family portraits) in their cubicles, but would be startled by a wedding photo with two women or two men.

If lesbians are allowed to make even non-sex-related reference to their sexuality, others might have to think about how lesbians are real people who might be near them at work or in the supermarket or in the gym's locker room. And if lesbians are seen to exist, then others might have to acknowledge how their actions have harmed or hindered other actual people.

Nutshell: Fairies don't make them think about their privilege. They make the "credibility" argument not because they don't believe in lesbians, but because they don't want to believe that they have any responsibility to address cultural inequality.