I guess one possibility is that if you say you ar bisexual, people assume you are not monogamous. That indicates you will have more than one partner, maybe several. Personally, I don't assume anything cause it doesn't matter to me whether you are promiscuous or not.
Do you know how certain people think that gays wants to sleep with everyone of their gender? Now imagine if you meet someone that is attracted to both genders - those people will automatically assume you want to sleep with any person that breathes.
Just ask them if they want to sleep with every woman/men out there. Most of them will shut up quickly.
I don't think it has to do with a person being bi, any more than if a person tells people they have been with a number of sexual partners. If a person implies they are promiscuous, they will be be perceived as perspicuous.
I can always cut off my penis, if that makes you easier to catch.
Then again... Wait... Oh.
Why do people, especially women, when they find out you're gay, they automatically assume you're only gay because you "haven't been with a woman like me" and continue to make unwanted sexual advances, or they get offended that you don't find them attractive? Or they throw a hissy fit when they realize flirting to get their own way isn't going to work on you?
this has happened to me in the bar a number of times. "OOh you're not really gay, you just don't find me attractive!" or "Well you just haven't been with ME yet!" or "How can you be gay? You're not flamboyant!"
If a straight guy tried that with a lesbian, he'd be in a body cast quicker than you can shout "MISOGYNY!!!!"