[Ghoulish Delight]

I think it's really just the fact that no one was really paying much mind to female homosexuality. While the idea of same sex female marriage and relationships shares similar status as between males, it's well established that same sex female sexual encounters do not share the same stigma (and are supported by the same bible that fuels the stigma against same sex male relations). So for most of the history of the words, "gay" and "homosexual" referred to male homosexuality by default, with a cursory nod towards, "Oh, well, yeah, there can be female homos too I guess". "Lesbians" arose because there was a need to specifically distinguish that you weren't JUST talking about males.