So like are you into threesomes?

This is one of a number of questions I discussed with a female friend recently. Several of my friends would identify themselves as bisexuals and this is one of a handful of questions that seem to automatically come up when you mention you're bisexual.
It's so surprising to me that bisexuality is so often associated with polygamy and general promiscuity. They assume because you like multiple genders you must like multiple people generally. Hence the question 'So are you into threesomes?'.
Another ridiculously common question is 'so which do you prefer boys or girls?'. This one really irritates me; the whole meaning of bisexuality is that you like both genders. If you had a preference that would probably mean you weren't wholly bisexual.
I really dot understand why people think its perfectly ok to ask inappropriate questions or interrogate you about your sex life because you're bisexual. They wouldn't ask these questions of a straight person or even a gay person. Its hugely frustrating to be treated in this way just because you're not considered to be the 'normal' sexuality.
At the end of the day comments like 'bisexuality doubles your chances of a date on a Friday night' or 'oh you're just greedy/indecisive' might be funny in bisexual circles but to have your sexuality constantly belittled or be told it's 'not a real sexuality' is cruel and horrible. My own mother said she doesn't 'believe in bisexuality'. I guess my frustration stems from not understanding why bisexuality is any less valid than being hetero or homosexual.
Attitudes like this annoy me almost more than people being genuinely offensive; using terms like 'bent' or 'fag' or 'dyke'. These are words that a lot of the gay community try to re-appropriate or take on for themselves. A handful of bi friends of mine jokily call themselves the 'dyke crew' though I used to despise the term 'dyke' growing up. It's like anything else; whether short, tall, thin, fat, ginger etc. its got a lot to do with owning who you are and what you're about. I guess it kinda saps your self confidence a bit if people constantly belittle an integral part of who you are. Its equally difficult if they think its soooo fascinating and seem to consider it your definitive feature. Being 'the bisexual one' isn't how you want to be seen; just like you don't want to be 'the tall one' or 'the fat one' or god-forbid 'the ugly one'; even 'the hot one' isn't what you want. No one wants to be defined by a singular feature. I wouldn't mind being ' the quirky one' or 'the funny one' but we all have much more to us than any one thing-no matter how big a part of us it is.
I dislike anyone who makes assumptions about you based on one trait or based on appearances but especially inappropriately comments or asks questions they have no right to. People process things in different ways but some people don't realize how their comments can be perceived. Frustration I guess just motivated me to write something about this...