It is true that one's own sexual and romantic attraction cannot be controlled, whether it is directed toward the same or opposite sex. It is also true that one cannot control a lack of attraction to either sexes. However, one's "sexuality" cannot be removed from the body and studied like one's internal organs or physical sex.
You, who are reading this, certainly have either a sense of sexuality or lack of it. You might apply a label to this: straight, gay, lesbian, bisexual, asexual, or even a more specific label. You might not have a label for it at all: "I am who I am." This is all normal.
If you woke up tomorrow, in a world where nobody has physical sex nor gender nor preferences for sexual relationships beyond hair colour, what would that label mean? In reality you would have to restructure your own "sense of sexuality".
Sexual orientation exists as a way to describe one's own desires in relation to other humans. There is the societal (and in some ways biological) default—cissexual heterosexuality—and there are ways to describe being different to that. It is also highly dependent on gender; if you are a man, your attraction to women is seen as "natural". If you have this same sense of attraction as a woman, this is no longer the case.
Much like cisgender men do not see themselves as having a gender, and White Americans do not see themselves as having a race, a straight cisgendered individual is unlikely to see themselves as having a sexual orientation. They are simply "normal".
The idea of a gay or bisexual or otherwise queer person being born this way is a nice shorthand. In fact, many people both in and outside LGBT communities have taken this phrase as gospel. It's not the worst thing in the world, but it also introduces the idea of a "gay gene"—which, in reality, does not exist at all.
There is also the case of those whose identity has genuinely changed. Much like preferences towards food and music and culture can change, romantic and sexual perferences can too. It's rare that they are as significant as desiring a different gender as before—they may just have to do with what a "happy married life" looks like, or the desire for marriage at all—but these changes still occur in some people.
If a man who has been straight for his entire life finds himself, now fully immersed in his adult life, falling in love with a man for the first time and has surprised himself with this: is it fair to say that he has always been attracted to men, or secretly closeted? I think it's fine to mention that he was straight before, and now he might not be.
He might even still be straight, generally, but have a strong desire for this one specific man. I don't think it's something to be discounted, or something to worry about.
There is also the matter of a gender identity change!
Someone may consider herself a lesbian for years before transitioning, and after having a and completing the transition, he may consider himself a straight man. Does this mean that, with all his time spent in lesbian communities and time lived as a lesbian, he was never once a lesbian? And is he not straight now, years into living completely as a straight man, with girlfriends and maybe a wife to back it up?
All this and yet, on social media, if you take a wrong turn all you will see is discourse surrounding the way people treat sexuality: "sexuality is fluid" is often taken as an attack on someone's individual sense of sexuality, but the opposite idea does not take into account the wide scope of human experiences, either.
In theory both sides of this style of discourse are completely meaningless: your sexuality is real as long as you give it importance. In reality there are a lot of contributing factors.
After all, anybody can say that gender isn't very real; it takes a much bolder streak to keep that worldview consistent with regards to the idea of sexuality, too.