This whole thing seems rather old-school to me. I've been reading Boots of Leather, Slippers of Gold, a history of the lesbian community in Buffalo, New York from the 40s to the 60s. One thing they talk about there is how in the 30s and 40s, sometimes femmes weren't really considered lesbians, and only butches were the "true" lesbians.

This is a reoccurring theme in the history of how people conceptualize gay people in pre-modern times, both with gay men and lesbians. Gender nonconforming people who were interested in members of their own sex — they were the indefinably "other" ones, the "sexual inverts" or whatever. But their gender-conforming lovers weren't really considered gay, and often have straight relationships as well. By modern standards, we'd probably call a lot of them bi. (Although—given the gender-conforming gay people I know personally—I suspect a some of them probably were fully homosexual, the hetero sex they had was something they were pressured into.)

This strikes me as the modern reincarnation of that older idea.