The saying goes, what someone doesn't know can't hurt them, depending on your actions towards them. Now if you know are bisexual, but have no intentions acting out with other women, there is no need to tell. At your age, I would be very careful what you tell guys even if they are your boyfriend, they could use this information against you, and you could be a target for bullying at school. If you were an adult, it might be a different story. Please think very think very carefully what you say to anyone about yourself, cause they might just use it against you later. If people find you like other girls at school, they could bully you for being gay. Teenagers that are gay, in schools are big targets.
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The other thing is, she could be a target for bullies at school and college, and her boyfriend could use it against her, if she dumps him for whatever reason. I think its a very dangerous thing to do at her age.
@Maddenis123 I agree, I wouldn't recommend saying anything about it unless asked.