Adults choose who to have sex with, how to have sex, where
they have sex, and the frequency with which they have sex.
Libidinal desires may exist beyond our control, but we do very
much have control how we act upon those desires externally
in the world with other people. To put it personally: “I just
don’t fall into a vagina and stay there,” is how I jokingly explain
my belief that sexuality is a choice. I am drawn to women
and will happily talk about scissoring and other e
xquisite lesbian stereotypes, for sure, but there’s not an inescapable
magnetism forcing me to leech onto women.
— Marcie Bianco, "Yes, my sexuality is a choice:
Why I reject the 'born this way' narrative," Salon.com
Marcie Bianco is correct to stress human choice with sex, but we need to note more carefully how that choice is conditioned and restrained. So let's review some basics.
Phenotype is all that can be observed about an organism, including in animals our behavior. In good materialist mode, and remembering Karl Marx's admiration for the work of Charles Darwin — and oversimplifying — phenotype is the product of the interaction of genotype with the environment over time, with the environment for humans starting in the womb and most immediately including the language and culture in which one grows up. Possibly flipping from materialist mode (for a moment) most of us would recognize the possibility of adding free will to the equation, and I'm very glad the article uses the idea of free will and choice and the positive sense of agency. (The negative sense is what Stanley Milgram talks about as "the agentic state": when one feels oneself totally the agent of another or others, thereby lacking positive agency.)
In non-mystic, materialist mode, if a member of the human species does something, whatever that something is has its roots in the human organism's biology. Given the huge number of possible genotypes and indefinitely humongous possibilities for environmental (cultural, linguistic, personal) histories, human phenotypes will fall into a broad and dynamic range, including sexual desire and even more so including sexual behavior. A. C. Clarke suggested an axis for homo/hetersex with relatively few humans at either extreme. So long as we recognize other possibilities besides homosex and hetero-, Clarke is correct.
Given our desires, I'll get both mystic and rigorously empirical and say I feel free and believe in choice. Given the desires that have developed in us as we've developed from conception on, we're free to act on them. With no exceptions I can think of offhand, those of us with sexual desires want to act on them, whether trying to get sex with everything that breathes and moves (and a few things that don't) to the "null case" of no sex asexuality. And we legitimately feel socially constrained if forbidden to act on our desires. The question then becomes what the State, culture, society, and/or family can and should do to limit acting on desires, however we got them. What may we be justly told we may want to do but damn well should not do?
If heterosexuals were told that they may've been born that way or had developed that way by a young age but should not act upon their desires — see Joe Haldeman's Forever War for a thought experiment on that subject — heterosexuals might very well feel imposed upon and victims of injustice. Ditto for other varieties of desire. We might very well tell pederasts, however, that nah, however you folk got that way, going after little children is unjust exploitation and you should choose not to do so; and if you do you'll face really strong "negative reinforcement" from their societies and the State. (I use the old Behaviorist "negative reinforcement" so people can agree with this conclusion even if they reject the idea of free will.)
I don't think we choose our most basic desires or our underlying orientations — or who our parents were or when and where we were born; so in many respects our various ways of life are constrained. However, there are lifestyles and style definitely implies choices.