Sexuality is defined by which gender you prefer MOST.
Bisexual would be stating that you're aroused and lust after and enjoy each gender the exact same amount.
But becuase of societal implications, people can't be Hetero and occasionally enjoy another guy, becuase of the complications with communication.
So for those with a 50/50 rating on each gender, bisexual would be appropriate, but it is also a title used to communicate general comfortability with swinging either way. Without this title, Hetero or Gay is usually translated to a very black and white viewpoint on who you' like to bump uglies with, without any gray.
Maybe some day, when humanity has become so hedonistic that it completely sheds its natural evolutionary, societal traits, the word bisexual won't be needed anymore.