I usually figure that eventually the feminine one of the two is actually bisexual and will probably leave the relationship for a man eventually. It usually only takes the feminine one meeting a man she can connect with for the hetero-switch to get turned back on.
It’s not that the man switch is off, they see a ‘man’ they want in a woman who looks like a man. The hetero is still on, they are attracted to a masculine, male-looking woman. And you’re right, if they find a guy that they are more attracted to, they could very well go for him and dump the butch lesbian.