While I believe sexual attraction can change over time, you seem to be equating who a person is with at the moment to their sexual orientation.
If a person is attracted to only the opposite sex, they are heterosexual. If a person is only attracted to the same sex, they are homosexual. If a person is attracted to both sexes, they are bisexual. Those are the basic definitions.
Things get more complicated with differing gender identities, but I'm trying to keep it simple both because it's easier to talk about and because I'm not well informed about a lot of recent terms for sexuality and gender identity.
No, I said "At what point do they become "gay"? If they like it but went back to their girlfriends when they got out, were they ever gay?"
There are straight guys that went to prison and came out gay and stayed that way. At what point did they become "gay"? Like I said I don't care what people call themselves, a man is always a man and homosexual acts are what they are. No need to mind read.
The point at which a person is not attracted to the opposite sex but is attracted to the same sex is the point at which they are gay.
It's hard to give a definitive answer about specific individuals because no one else can know what they are thinking and feeling. It's not about a particular act, it is about that person's attraction to the genders, and only that person knows the truth about who they are attracted to (if they even realize it themselves).
You seem to be looking for an answer along the lines of "When you have intercourse with a member of the opposite sex you are gay", but that isn't how it works. Again, if a person is attracted to members of their own gender but not to members of the opposite gender, they are gay, at whatever point that is true.