Short version, I was 17, lonely and wanted a relationship, and realised that I didn't care what gender my partner had, as long as I had a partner.

Long version in the spoiler:
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At around 14, when I started having interest in "more than friendship" relationships, I very shortly thought about if I was a lesbian (I still thought I was a girl then), but quickly came to the conclusion that nah, I wasn't, because I wanted a boyfriend.

At around 15, I very shortly thought about if I was bi, but because I wasn't interested in anyone specifically, and society generally says that girls are supposed to only want relationships with boys, I quickly forgot about it again - after all, I wanted a boyfriend.

At around 16, there was a girl at school who was really pretty, and I thought "if I was a lesbian, I'd totally have a crush on her". But because I wasn't a lesbian, I never thought anything more about it. (Also, she turned out to be... interested in very different things than I? Like, she was still very pretty, but intellectually/mentally really not my type.)

At around 17, my group of friends had sort of a running joke that I was an elf, one friend was a vampire, one a werewolf, and one a zombie (all girls, I went to an all girls school). Werewolf friend wanted to change religious denomination, and here you have to go to the civil register office (or however it's called in English) to do so officially, and she asked me to come along for moral support. This office is the same where you go to legally get married, which led to our mutual friends joking about us actually secretly getting married, and we went along with it because it was funny.
A bit later, vampire friend and I had to do a roleplaying/argument thing in English class - we had to pretend we were a couple with a child and had to discuss something about how we wanted to raise the child. Naturally, my group of friends again made a joke about it - that I had cheated on my "wife" (werewolf friend) and now had a child with vampire friend. (We decided that this had to mean that I was either a guy (which I didn't like) or elves (and therefore I) had to be hermaphroditic (an option I liked. Vaarsuvius might have something to do with this.) Of course they were both "angry" at me for "lying" to them, so I replied "That happens when you date an elf!", and because of course I didn't want to be an evil cheater, I made something up about elves naturally being polyamorous and me just not understanding human (or werewolf and vampire) customs, and wrote a little story about it. My friends liked the story, so I wrote more, and started researching – about hermaphroditic animals, which was sort of a stepping stone to me finding out about all kinds of queer stuff - intersexionaliy, polyamory (I had no idea this actually existed) and trans stuff (which I also had never heard of before), mainly, but this also brought the option of bisexuality back on my radar. And I realised that the thought of being in a relationship with a girl didn't bother me at all, and actually seemed really nice. (Around this time I also discovered the LGBTAI thread here, which taught me a lot.) And because I was still lonely, and still wanted to have a relationship, I decided that yeah, I really didn't care what gender a partner had, so I started identifying as bi.
(Another thing was, that I have imaginary friends, and I daydream excessively, and around that time I gained two new imaginary friends, a girl and an (agender) elf, while the IF I had had for 3 or 4 years at that point (a boy, and yeah, he absolutely was my imaginary boyfriend) mostly went away - he still stayed around for a while, but after some time he was gone completely. The girl didn't stay around too long, but the elf is still here, 5 years later, and I'm really glad I have hir.)

Anyway, a few months later I developed a crush on werewolf friend, thought she might like me to, asked her out and was rejected, I was heartbroken for a while, but I moved on and we are still friends (even though we don't talk as often anymore, cause we live in different cities now and are both busy with university). Oh, and I also figured out that I'm genderfluid some time after that – those of you who were active in the LGBTAI thread around 2012/2013 might remember.

A bit later I also figured out that I'm (grey) asexual, which was actually way harder than realising I'm bi, because I really didn't know how sexual attraction is supposed to feel, and while I can imagine myself having sex with a partner, I don't really find people "sexy" or "hot" - I don't really see a difference between, say, a beautiful actress and a beautiful sunset. I mostly ID as grey-a (instead of just ace), because I don't want to count out the possibility that I might find someone attractive in the future, and I'm also not 100% clear what my feelings for werewolf friend were - how much of them were romantic, how much just really intense platonic friendship feels, and how much might have been sexual attraction. I'm adding the stuff about asexuality here because the process of discovering your orientation tends to be similar for aces and bi or pan people - "I feel the same for all genders" - and just the specifics (I find all genders attractive vs I find no genders attractive) are different (there are a lot of ace people, especially aromantic asexuals, who ID as bi or pan before they find out about asexuality/aromanticism - "I'm not straight, and I'm not gay, so I must be bi").