Okay then. So I’m going to respectfully disagree
First of all, Science can’t prove anything. The goal of science as a whole is to understand how the universe works, and the number one principle is to never accept something as permanent fact. Just because something lies outside our current observations doesn’t mean it can’t happen ever. Laws of physics and nature are only called “laws” because we are yet to observe things that falls outside it. The rest are theories built upon observations that have known exceptions or limitations. So while Science is related to finding the truth, said truth is never permanent.
You say that humans only have either XX or XY chromosomes, and those cause specifically either the male or female gender. Two things wrong with that: first of all, intersex people exist, and they can have XXY chromosomes among many other combinations. Secondly, even if intersex people did not exist, they only define physical characteristics, and while there is a strong correlation between the phenotype of the sex chromosomes and gender, we have several examples of peoples throughout history whose gender differ from their physical traits. Tell me if you’d like to hear more about this.
And for sexuality, even if we go by your assumption that only male and female genders exist, there exists another: Asexual, or one who feels no sexual attraction at all. Or heck, even Demisexual, one who only feels sexual attraction towards those with which they have an established bond. Though some see that as a subcategory of Asexual, so I won’t harp on you for that