Part of me doesn't exist? Brother problems.

So, I'm going to go on a little rant here about my family in general... But most specifically, my brothers.
One thing about myself, that I have always known- is that I'm attracted to girls. But I am also attracted to guys. So naturally, as of now, I identify as bisexual.
However, I struggle with that because my grandma, my mom, and my brothers all think that it's a phase that I have on and off. (While somehow my dad just found out recently, and trusted my word immediately, no questions asked, and no criticism whatsoever- which is strange for my situation, because I am not close to my dad at all.)
My first story is about my step brother. (Side note: both of these boys are younger than I) Every time there is two girls fighting or getting intense emotionally on tv, he always says- "KISS!" It just makes me cringe- I don't know why... Anyone else have someone in their life like that? No matter what I tell him, he still doesn't stop... (We're all young adults- out of high school- so you know.)
Alright, now here's the big story:
One thing I grow tired of, with my blood brother, is his random proclamations that bisexuality doesn't exist. He says, it's all black or white- either you're gay or you're not, or it's just a phase. He also seems to think being gay is a choice- of course we never get into that subject- but I'm sure his responses would make me grit my teeth.
I love him dearly, really, but today he went and did it again, and it bugged me more than normal, because my grandma chimed in with her discomfort.
While I was visiting with him and my grandma about my early high school years, he said, 'you had friends who were girls? I thought you hated girls.' I said, 'no, I like girls, I get along with them just fine, sometimes better than I do with guys.' he responds, 'oh yeah, I forgot about your lesbian phase.'
First of all, I have never dated a girl, or been with one.
And of course, here's most peoples' response to that: "then how do you know you're into them?" To which my response is, "how did you know you liked the opposite gender before being with them?" 'I don't know, I just did.' Exactly... people are born that way.
Anyway, so I told him it wasn't a phase, and that I'm attracted to girls and guys at all times, and that I'm with a guy right now, because that's who I'm in love with- if I get with a girl later in life- that doesn't mean I was having a straight phase while I was with my boyfriend(s) or that I would be having a 'lesbian phase'- it just means exactly what I said it means- I am bisexual. My grandma cringes and is like, 'enough! that is way too much information!' Basically, after sitting there for a few moments, trying to explain that it's natural for me, and things aren't just black and white- they threw some hateful words, and my grandma agreed with my brother- that I was just having phases, and that I hadn't found the right guy- and blah blah blah and I just went upstairs with my dog.

Is anyone else struggling with the invisibility cloak of bisexuality?
I feel like I'm totally invisible in the LGBT community too- I haven't really found a group that doesn't sort of shut me out for having a boyfriend- like they assume I can just chose the "easy" life. I don't chose who I'm attracted to- or who I wind up getting the chance to date! :bang:
But here I am! I can't even get help from the community that is supposed to support me. I really hope things get better when I go away to college this next year. It's really something I struggle with everyday. My boyfriend is still feeling the facts out too, but luckily, he's very respectful of the fact, and doesn't ask me rude questions, or make me feel like less of a person. He just accepts that other piece.
As for the family, we're still working on it. I just feel like I have to constantly remind the people around me that I'm a part of the community they put down so often.
It just bugs the crud out of me. I don't know what to do. :icon_sad: