Excepting at the start of a sentence, surely the good folks of Lesbos can distinguish themselves from the good girl-on-girl lovers through the use of a capital or lowercase "L". Hence: Lesbian - resident of Lesbos; lesbian - woman who lurve woman.
Problems may arise at the start of a sentence, whereupon you do what anyone else does and go by the context. Even better if you're a Lesbian lesbian.
While we're at it, we could call welsh ones Llesbians.