If they enjoyed the homosexual experience at the time, then of course I believe they're bisexual. Just because they prefer to stick to one gender after gaining some experience, hardly makes them heterosexual. This would make for a great Monty Python skit.. "No sir I'm not homosexual, you see that was yesterday!".
If they switched once, it stands to reason they might switch again. This is not true of your standard definition heterosexual. Notwithstanding chance encounters they regret afterwards, which is a completely different kettle of fish.
