OP, your misconceptions are all wrong. Almost no-one is 50-50 in attraction any more than people are 100%. The vast majority of people are along the lines of 59/41, 96/4, 68/32, and so forth. The ratios, which don't mean much, keep changing depending on the day, the mood, the year, recent experiences, who you met last night, and so forth.
Bisexuals are those who can ENJOY having sex and/or sexual fantasies about both genders. It does not mean they have to screw everyone. Bisexuals are capable of monogamy. How is this possible? The same way monogamy is possible among het's and homo's. Anyone who has ever been in a monogamous relationship knows that one person does not satisfy every desire, that there is temptation and fantasy everywhere, and that sometimes monogamy seems unfair. That does not mean monogamous people are bad or untrue for thinking such thoughts. They are only bad when they break a commitment to fidelity. Whether one is bi, het or homo is not the issue.
Sex researchers tell us that sexuality -- of which gender orientation is only one of MANY parts -- is not precisely fixed. It changes with time. Don't box yourself in. Don't box others in. Be open to possibility.
Suggesting bisexuality does not exist is as oppressive and misguided as saying homosexuality does not exist. For both orientations, it has been pointed out that it is just a phase someone is going through, that is is unnatural and impure, and that these poor individuals are just confused.