Much like everything it has the misfortune of touching, the gender binary has regrettably infested every aspect of how we label sexuality.
Society forces us at birth into a binary set of gender roles solely because of our genitals, which conditions us to see people from the other gender as fundamentally different. Gender is seen as some innate truth independent from the constantly fluctuating reality that forms our Self through language, laws, norms, and experiences.
The concept of “biological male/female” is just the gendering of the body. People’s bodies contain a multiplicity of characteristics and capabilities. Grouping the body into one of two binary categories is a social, cultural, and political act.
No body type or its genitals is inherently “masculine” or “feminine”, so sexual distinctions of “gay” or “straight” being held on the basis of being attracted to a certain body type is reinforcing the gender binary. In simpler terms, sexuality is influenced by how we perceive the constructs of masculinity and femininity. When a man claims to be attracted to other men, he means that he is attracted to his preconceived notion of what a man is — his notion being that a man is a man because of his physical body type. What does this make of trans men? Or of men that do not fit the societal criteria of masculinity? Or of queer, non-male individuals that do? The label of “Gay” becomes arbitrary, because “Man” as a category is arbitrary. It is likewise for the category of “Woman”.
I’ve seen many examples in which people of a particular sexual label learn that the gender of someone they are attracted to does not align with their sexual identity. To them, this is supposed to be impossible.
However, the immediacy of beauty and passion makes identity categories insignificant and shows the fragility of the gender binary. The spirit of queerness is to embrace love beyond artificial restraints. Gender norms are weak, arbitrary, and impermanent masters that are not worth our obedience.
On a side tangent, no one is born cis or born transgender. We are made cisgender by the categories inflicted upon us at birth, and we are made transgender when we break free from those arbitrary categorizations.