I prefer thinking of them as experimenting. They aren't sure WHICH they are sometimes. They don't go back to that lifestyle as far as I can tell, but, who knows? I'm only speculating.
I think sexuality is more of a continuum than a binary. The majority of people are on the "completely straight" (would never consider engaging in homosexuality) or "almost completely straight" (would only consider engaging in a homosexual act in very limited circumstances, such as experimenting in college)end of the scale. A small minority is on the "completely homosexual" end of the scale. Finally, a slightly larger minority is flexible in their approach to homosexuality, in varying degrees. I knew a guy in college who was involved with men and women in a 1:2 ratio, more or less, for example.