Butch / Femme dynamics originated in the 50's bar culture as a way of survival - GNC women (gender non-conforming) women couldn't get jobs. I.E. if you had your head buzzed, wore pants or other transgressions etc - so they often found themselves with femme partners (not just romantically) as a necessity of survival.
In turn butches acted as "guardians" - being visible so drawing attention from homophobes, and beating the shit out of them when necessary*. Being Butch or Femme is much more than just how you dress, it's a whole attitude and way of life (at least in the west where it originated).
Also it's not heteronormative. Two women together will never be heteronormative lol.
*see: Storme DeLarverie, who fought police outside Stonewall for 10 minutes while handcuffed, inciting the riot.