Yeah, I think it's probably a different reason for every person. Some people might not want to do anything with a guy/girl, and have no desires to up until this point, but don't want to make a liar of themselves in the future by saying "never", so add on that 1%.

Remember, there is a difference between sexual preference and sexual orientation. My orientation has always been homosexual, but when I was younger, I fought against that, and tried to be straight. My active sexual behaviour was heterosexual, but I was not. The same is also true of many straight guys who mess about with guys. They're still straight, but make a choice to engage in pleasurable activity with other men. It doesn't make them gay or even bi, if they're still only properly attracted to women.