I always thought I was straight.. never questioned it (I'm 21). But I met a girl who I'm pretty sure I have a crush on. What are the odds I'm bi if she's the first girl I've thought of like that?

You don't have to be bisexual, sometimes there are people who choose to identify as straight but have one or two same-sex crushes which is okay, if that makes you feel more comfortable. You just have to assess and see whether that feeling is a crush or admiration, and whether the feeling is romantic or sexual, because some people are attracted to same sex partners romantically but not sexually and vice-versa. Though sometimes it is easiest to just drop labels and let yourself feel and love however you do.

If you feel that you are bisexual, you should simply try it out. We all do feel a bit confused with our sexuality at some point in life, and you don't even have to label it. Start with observing how you feel around other girls/boys, and remember you are much more that your sexuality Just accept how you feel, you never have to prove anyone the reason. And if she's the first girl then, she's really someone who grabbed your attention in a more connecting and relatable way, cause you feel good about being with her irrespective of her gender. If you are still confused just try it out, and see if you still feel that way or not.

It is likely you are bi if you also experience attraction to someone the same gender as you. Being bisexual does not require you to have crushes on many people of different genders. There will always be a first to things and experiences. It does not matter much that she is the first girl you have felt romantically attracted to. What matters is that you do experience romantic attraction towards her. With that said, I think it is safe to say that as long as you have experienced ronantic attraction to more than 1 gender you can identify as bisexual.

There's no specific number of crushes you must have had to consider yourself as bi, what matters is who you are attracted to. You can still be bi even if you have a strong preference for one gender, it doesn't have to be 50-50. So maybe your strong preference for guys lead you to think of yourself as straight, and this experience is teaching things about yourself that you didn't know before. Either way, what truly matters is your happiness: if you like this girl, go for it! And remember, you can also decide not to label yourself if at the moment you prefer to just live your life and not think about implications.