The concept of sexual orientation has only been around for 200 years. The decision that sexual orientation should be based on one's gender is arbitrary and vestigial. This becomes obvious when we consider the existence of nonbinary people.
Is an enby exclusively attracted to women a lesbian? Did your brain reflexively attempt to consider what that enby's Assigned Gender At Birth was? That'd be bad. The proper conclusion here is that anyone who identifies as a lesbian is a lesbian, including transmasc people.
The synthesis here being: Sexual Orientation has nothing to do with one's own gender.
Hmm.
I would also make the argument that enby's were simply not considered at all when constructing the current sexual orientations that are well known. Women have orientations, men have orientations, but NBs have no orientations? Doesn't make sense.
It makes sense to me that there should simply be NB orientations. If we're considering NB a gender in and of itself that is. But it can get more complicated if we're considering NB an absence of gender, well that's a different issue altogether, their pronouns are also not technically a gender either but simply a convenient linguistic tool. It's also very complex if you're considering NB as a range of gender variables.
The issue linguistically seems to hinge on what one person's definition of NB is, which definitely varies among NB people themselves? At least in my experience with the friends I know offline.