You just haven’t found the right person yet

It’s unlikely that someone who identifies as straight, simply hasn’t found the right person of their own gender yet. The same applies to asexual people. It just covers people of any gender. I’m not gonna say it’s impossible. It could happen that someone, who identifies as asexual, develops sexual attraction to someone. So let’s say that happens, how would that story continue?

Option #1
They start a sexual relationship with the person they’re sexually attracted to, break up at some point and the ace person continues their life without sexual attraction. The person they felt sexually attracted to is merely an exception.
Would that mean they aren’t really ace? No. They could identify as gray-ace as well as ace, but one exception hardly makes someone allosexual.

Option #2
They start a sexual relationship with the person they’re sexually attracted to and live together happily ever after without any loss in sexual attraction. The person the ace feels sexually attracted to is still just an exception, though.
Would that mean they aren’t really ace? No. They could identify as gray-ace as well as ace, but one exception hardly makes someone allosexual.

Option #3
They discover that they actually do sometimes experience sexual attraction to people, but only under special circumstances or only a few times throughout their life.
Would that mean they aren’t really ace? It suggests that they are some variation of gray-ace.

Option #4
They start a sexual relationship with the person they’re sexually attracted to, break up at some point and due to having their sexual attraction option unlocked, the ace person now experiences sexual attraction frequently and towards a variety of people.
Would that mean they aren’t really ace? Yes! It probably means they’re a video game character, because that just doesn’t happen in real life.

Option #5
They discover that they had a low sex drive or sex-repulsion, weren’t ready for a sexual relationship yet, are late bloomers or had any other “symptom” that could be mistaken for asexuality.
Would that mean they aren’t really ace? Yes, that’s what it means.
Those cases do exist. But they are exceptions and should be treated as such.

Asexuality isn’t something that needs to be fixed and asexual people don’t need to be treated like they can’t possibly have a meaning in their lives without sexual attraction.