I know a lesbian who has genital warts. She went to a PCP for a check up and mentioned some problems in the female area and the PCP said, "Oh well it's good that since you're a lesbian we know it's not an STD."
When GF and I got together we decided that we should both be tested for STDs. The doctors seemed to think it wasn't necessary at all because lesbians can't transfer STDs (and I've even had male partners before!) and kind of rolled their eyes while the did the tests. And this was on a university campus where when a straight woman comes in the first thing they think is wrong with her is either an STD or pregnancy!
Of course, it is true that women who have only same-sex contact have much lower rates of STD transmission, but it is still possible, and given that the medical community seems blind to it (and I've read studies that lesbians have lower rates of going to the gynecologist than straight women) I wouldn't be surprised if STDs go untreated more often in lesbians.