The problem with trying to gather statistics on the sexuality is that, a lot Gays are married to someone of the opposite sex. I have known about half a dozen Gay men that were married. Most of the time the wives seemed to have no clue that their husbands were Gay.
I agree with your observation and suggest that this 'flaw' is inherent to all studies that base thier terminology and observations upon subjective query that even if communicated clearly is still prone to paricipant deception and observant manipulation. As they say -keep it simple and objective.
If one freely chooses to participate in homosexual activity they are therefore based upon the activity a homosexual. They can stop being a practicing homosexual by stopping the homosexual activity.