You're assuming that sexuality is as simple as that. There are some women who have tried women because they're tired of men or whatever. Those are the stupid and immature ones. But most if not all of them genuinely like women.
Personally, I'm under the belief that once you cross the line into bisexuality after you've discovered yourself, there's no real turning back. Girls telling me "I used to be bi" always gets the

faces from me.
Over time people could truly develop feelings, both sexual and romantic, for people of the same sex, no matter what they did early on in their lives.