i thought that the whole point of social sexual justice was accepting that it wasn't all just a phase. so in this respect, someone saying that they can change their mind about their sexuality is pretty... odd at the very least, and dare i say it, triggering :p still, it's a very clunky way to say that the person is genderfluid or non-binary.
Part of social justice (a part I agree with) is that sexuality and gender identity are more complicated than the standard view of "men like women, women like men, nothing else under the sun". How much more complicated is a tricky question, I'm not getting into it. But anyway, the more complicated you admit gender can be, the more allowances you have to make for people exploring their gender and sexuality.
I'm sure trans people are not in general born knowing they are trans, gay people are not born knowing they are gay, etc. If we accept someone can change their mind from "I'm straight" to "I'm gay" or from "I'm a man" to "I'm a woman", it does not seem
a priori ridiculous to say that one can change their mind about being non-binary or whatever. That doesn't imply it's a phase, just that they are examining a larger gender-space.