Some lesbians, to be sure, seem to have butch ingrained into their sexuality. They will often tell you that they were tomboys, that they never wanted to do girly things, that they feel ridiculous in feminine clothing. I know heterosexual women like this too, though fewer (maybe because it is harder for straight women to express this preference?).
I think those lesbians were over-represented in the ranks when homosexuality was a very, very hard lifestyle choice. They are women who simply cannot live in any other way without destroying themselves, and so who chose to be very brave. I'm not saying there weren't femme lesbians before the 70s - far from it - but they were not such a high proportion of the lesbian community as they are now.
For most lesbians, I think butch identity swings in and out of fashion. Among the young peopole [strokes whiskers, sucks on pipe] the current aesthetic seems to be a kind of pretty, boyish androgyny - like Shane in the L Word. Definitely female but lean and sporty.
Most of my lesbian friends are in their 40s and 50s and look very conventionally feminine. I just look like a West London working mother, indistinguishable from the others at the schoolgate. You could meet me and I wouldn't for one second disturb any assumptions about 'lesbians are women with male hormones who like to be dominant in bed' because you wouldn't twig I am lesbian. Unless I chose to tell you.
Oh, and IME the link between what lesbians look like and how they act between the sheets is pretty weak.