There is nothing that prevents a person from deciding they want to experiment sexually, or completely alter their sexual identity, at any given time.
It would be a form of bigotry to proclaim that straight people can't choose to be gay of their own free will, and vice versa. It would imply we're not in control of our own sexuality, and that we lack free will.
And this is not a support of the bigots that proclaim that being gay is always a choice and that gay people should just stop pretending and change their minds --- those people were always in the wrong, their argument was always vile. Sticking to the: gay people are all born gay, premise, is a defeatist response to that bigotry, it's a very very poor defense when a defense is not needed at all.
The proper position is: if I want to be straight or gay, it is my choice, period. If I'm born gay, that's fine; if I'm born gay and want to be straight, that's fine; if I'm born gay and only want to be gay, that's fine. And so on.