I think you seriously need to take a step back and remember that your daughter is only in 7th grade. Now, try to remember what you were like when you were in 7th grade. Did you really understand what it meant to have a crush on a boy or have a boyfriend?
When I was in 7th grade, I remember just beginning to think boys were cute. I remember having a crush a couple of times. I remember being a junior counselor going on a "date" with one of the other junior counselors (lunch at McDonald's!!!). Wow!
I don't think I fully comprehended that some people actually were gay. Words like bisexual and transgender didn't really exist. Or if they did, I was unaware.
You are absolutely right that the media, the internet, youtube, twitter, etc., have all introduced this generation to things far earlier than we were introduced. It's important to remember that your daughter is still way too young to fully comprehend any of this, much less know her own sexual identity!
Your job, is to calm down! Listen to her. Be supportive. Love her. Know in your mind that this is only the beginning. She has many years of friendships and relationships and so much to learn about herself. She doesn't need you to freakout and tell her that she's not allowed to date and that if she decides to "chooses" this she has to be transparent with her friends and with their parents.
Telling her she's not allowed to date is a huge mistake for a few reasons. First, she's probably not even interesting in dating. She's hormonal and going through all kinds of changes and feeling that she may not fit in and just trying to figure things out. Basically, she's a typical 12 year old. Second, it makes dating "forbidden fruit," which can honestly make it more attractive. Finally, it tells her that she should want to date, which she probably doesn't actually want to do, and causes her to think that something is wrong with her for not wanting to date. Telling her that she is not allowed to date is honestly just adding fuel to the fire.
This isn't a choice she is making. It isn't as if she's seriously thinking to herself, "Hm, I could decide to be attracted to boys, or I could decide to be attracted to girls. Hm, I can't choose, so I guess I'll just be bisexual." It doesn't work that way. If you are attracted to people of the opposite sex, the idea of being with someone of the same sex is not at all appealing. It works the same way if you are attracted to members of the same sex. The idea of being with someone of the opposite sex just doesn't do it for you. This isn't a choice, and it's cruel to try to scare her into thinking it is.
If she is bisexual, why on earth would she have to be transparent with all of her friends and with their parents? Are you afraid she's going to hit on all the females that she knows? Did you hit on every guy you knew growing up?
Growing up is hard enough. Junior high is a difficult age. Your daughter does not need you to respond this way. You are only making it worse. Please just love her! She really just needs you to love her!