whats so *BAD* about being bi?

People say you have to pick a Man or Woman in your life. They say you CAN'T be in-between, and i would like to know, WHY the f**k not? Is there a LAW saying I'm not aloud to be bi or something, You cant tell what i cant or can be. Only my parents can do that, last time i checked your NOT MY MOM/DAD.

-Tammy