*Puts on professor hat* Ahem. Let me introduce you to the wonderful concept of phenotypic plasticity
. Basically everything we are is, in some sense, genetic. We aren't evolved to be netizens, for instance, but our pre-civilization ideas about fairness, reciprocity, and punishment dictate how we respond to trolling. The degree to which environment can change how our genes express a trait (called a phenotype) is called plasticity.
The classic example is English ivy. In the shade its leaves look like this:
while in sunlight they look like this(the shape, not the disease):
This is because the lobes allow the leaves to tessellate and collect all
available light in the understory, while the rounder leaves are more efficient for collecting light in the canopy. Ivy has to have genetic programming for both
shapes (and intermediates) because it's a climbing plant and has leaves in both the canopy and the understory.
The human brain is a classically plastic organism. That's why, whenever I hear of a new study finding differences between men's brains and women's brains (or between straight and gay brains, or any of the other mostly trivial differences neuroscientists like to look at), I don't go OMG teh wimminz r different from us menz and tear up my feminist membership card. It's at least as likely that twenty-plus years of growing up female has changed the brain. In fact, the very fact that you're capable of learning demonstrates its plasticity.
So, what does that mean for sexuality (and gender identity)? I dunno. From what I understand from twin studies, it's unlikely that sexuality is dictated entirely by genes. It could be that everyone
has the same chance of turning out either gay or straight or bi (there just isn't enough research on asexuality to say anything at all, yet), and that environment dictates the outcome. Also because of twin studies, I think that's unlikely as well. The reality is probably intermediate. Everyone has a genetic bias toward some portion of the spectrum, and then environment shapes it further. A hundred years ago, the shame of sporadic same-sex crushes I've had may have either magnified them so I'd end up more bisexual, or may have squelched them before I was cognizant of them.
Also, there's no such thing as a trait "in a vacuum." There's always an environment that shapes phenotype, even if that environment is "a vacuuum." Though typically that pheotype is "dead." Even a metaphorical vacuum is an environment: the closest two I can think of are neglect (for children) and sensory deprivation. In both cases, the phenotype that results is usually severe mental illness.
(A key note: just because a trait is plastic doesn't mean that it's changeable once it arises. Particularly for psychological traits, there's often a key period for plasticity and after a certain point, it's set in stone. Language is a good example: if a normal person doesn't have the opportunity to learn to speak by age 5 or so, you probably will remain nonverbal for the rest of their life, and your ability to learn new a new language without a significant accent basically disappears by the time you're 16. Likewise, asthma can be caused by a relatively germ-free upbringing, but by the time you reach adulthood, no amount of sterilization can cause it and no amount of grunginess can cause it. So wingnut pray-the-gay away types are still wrong and the APA is still right.)