Sooo… let’s say you live in a world populated by straight people and gay people. There are people who are sexually and romantically and emotionally attracted to members of the opposite sex, and there are those who are attracted to members of the same sex.
And, let’s say you just happen to be in that fractional group in the middle, and you find yourself attracted to members of the opposite sex and members of the same sex; not exclusively heterosexual OR exclusively homosexual. You’re bisexual.
To be straight or gay does NOT mean you want to have sex with EVERYbody in whichever group you are attracted to. Straight or gay, you find yourself attracted to individuals in that group of opposite sex or same sex people. And the reality is that whichever way you swing, you are not romantically or sexually attracted to most of the people you meet.
And when you are bisexual, it only means you can find yourself attracted to individual men and individual women at any given time. Just like with gay or straight, being bi does NOT mean you want to sleep with everyone you meet. You just happen to be attracted to a broader range of individuals.
So now, let’s say you are bisexual, and you start dating someone of the opposite sex. And you like that person, and that person likes you. And the other person is pressing for a stronger commitment, moving the relationship to the next level. And that other person says, “Please promise me you will always be straight now.”
The person who expects you to turn off part of your sexual identity does not understand how sexual orientation/sexual identity works. Who you are attracted to is NOT something you can just turn on and turn off.
Straight people do not stop being attracted to members of the opposite sex just because they are in a relationship. Gay people do not stop being attracted to the same sex just because they are in a relationship. So why should a Bi person be expected to stop being Bi just because they are in a relationship?
The issue is actually not who you are attracted to, but how mature you are in your relationship and whether you ACT on your attractions.
If you want to take a relationship to the next level, to make it exclusive, it is reasonable to say “I want us to be exclusive and not date other people.” That doesn’t mean stop being attracted to other people, it just means choosing not to act on those attractions, choosing to be monogamous and faithful to your partner.
And just because one person asks for exclusivity doesn’t mean they will get it, if the other person isn’t ready to do that.
Asking your partner or boyfriend or girlfriend to be exclusive and not see other people is reasonable, AFTER a reasonable period of time of casual dating and getting to know the person. Asking them to somehow flip a switch (a switch that does not exist) and stop being attracted to other people is both unreasonable and irrational.