Why do people liken being bisexual to being "on the down-low"?

Just because a man or a woman is attracted to members of both any sexual category doesn't mean they cannot happily be in a relationship with one person in one of those categories. Bisexuality does not preclude the ability to be monogamous or committed. And I don't think it's something that has to be addressed at the forefront of a relationship or a "talking," either. I don't sit down to have a conversation with my partners about the fact that I'm straight for the most part*, nor do I expect them to have that kind of a conversation with me, so why should that expectation change based on orientation? I hate double standards. I hate people who pigeon-hole LGB sexualities into little boxes of stereotypical behavior and then get mad when the world doesn't work that way. Yes there are gay men who are stereotypically masculine. Yes there are lesbian women who wear makeup and dresses and heels. Yes there are people who fall in love with a person based on their personhood, not their sex or their gender. There are even people who aren't sexually attracted to anyone. The are only two sexualities that I think warrant up-front discussion like that: asexuality, because sex/intimacy is generally expected at some point in a relationship, and polyamorous-ness, because I think people have a right to expect monogamy. But if your boo wants to be with you right now, does it really matter what other categories of people he/she is also attracted to? This seems like me not wanting to date a guy who has been with a woman of a difference race than me, or who is less educated than me, or any other social difference that might be a big deal. In other words, it would be ridiculous. 

*I am straight-identified. I want relationships with men. And while I love me some dick, I wouldn't run away screaming from the prospect of relations with a woman. End of story.