Hey, I think you've settled down now, which is good. I just wanted to say this is a horrible reaction to have to deal with from your mom, especially after you put yourself out there on the spot. But I think you can definitely make something out of this situation, especially if your father has a talk to her. I'm going to hazard a guess that the reasons for your mom's fear are a) an intuitive and probably socialised fear of the 'abnormal'/other b) fears for your wellbeing based on hate crimes/rejection, or possibly your mom believes that bi people are inherently more promiscuous -> AIDS and/or hell c) probably fears of appearance for herself as well, honestly. I'm not saying that justifies the reaction; in fact, it's all pretty horrible. What I am saying is that it is workable, because she may come to accept you over time if her reaction is centred around those things. I think if you do what Oglemi said and remind her occasionally, sit down to have a talk with her and keep her updated on your wellbeing and try to create an environment in which queerness can become normalised, she may be able to learn. Of course, remember your wellbeing comes first here, so it's probably best to drop it for awhile, because such discussions can be very difficult to deal with.So I just came out as bisexual to my mom. Let's just say that I'm pissed. When I told her, she kept telling me how she thought I was "choosing a dangerous lifestyle" and, honestly, just about the only thought that was going through my mind was "Fuck you, mom." My dad is tolerant, but my mom is not. But, unfortunately, my dad is still in Japan. My mom also told me how I "shouldn't make this known to everyone" because "some people will stop loving you over this." I was so fucking pissed at that I was so close to telling her to go fuck herself. She clearly doesn't fucking understand that it's not LGBT people pushing their families away from them, but rather, the other way around. And besides, if I stay in the closet, the bigots win. I can't let that happen.
Even though I lean more towards preferring women, I feel like I need to date men for a while just to spite my mom and pressure her into acceptance. I know her, and know for a fact that she will not move an inch on her ultra-conservative thinks-it's-a-choice views unless I fight adamantly to do so. I'd rather deal with lots of her shit now if it means she won't give me any shit later in life. But maybe it's just my current rage towards her talking, I don't know.
Re. the existing conversation: the thing with bisexual people is that there are many bi people who prefer their own gender. Plus while choosing to date or fuck someone is not involuntary, falling for someone mostly is, and for a bi person who does not prefer the gender they're 'supposed to' date, this leaves them wide open to falling for or being attracted to someone of their own gender. For most people, not pursuing the person they're interested in would lead to suffering.
Bi people do have the ability to 'pass' for straight, especially if they're in different-gender relationships, but whether the bi person wants to pass as straight or not is really up to them. Because to some people, it's a very important part of their feelings that suppressing feels painful, or because being stuck in the closet/assumed to be straight while people are being shitty about gay people around you really blows, or because, if it matters to you as a part of your identity, being 'dequeered' may feel degrading, honestly.
However, Kitten Milk, I definitely disagree that bi people who choose only to date one gender are antithetical to bisexuality/romanticism! There are quite a lot of queer people around who do this for various reasons. I can't speak for other people and how they manage their feelings, but dating is a choice that you yourself differentiated from attraction. Physical attraction can also exist while romantic interest doesn't, etc., which is a problem I see with defining everyone's preferences solely by their 'sexuality'