A women being penetrated is usually considered straight and man being penetrated gay but one action tosses a wrench in the neat engine, fingering. When a lesbian fingers another lesbian a women is literally inside of another women, how could that possibly be concidered straight? A women being penetrated has a grey zone of gayness. while you could say a strap on or dildo isn't part of the operating women there exist female penetration that is completely gay. You could say the issue is the fact that a dildo is usually model on the male phallus but there do exist many that do not, though it could be argued no matter how altered all dildo do have a penis as the original design base.
Still this issue is an enigma of a philosophical quandary that many academics will surly ponder for centuries. There is much to analyze and debate such as the fundamental issue of if women can even have gay sex since they have no balls that can touch.