I feel like most of my life was lived through movies and TV as a kid. So I knew I liked guys, I always had guy characters that I wanted to date.
Additionally, I always had girl characters I would get obsessed with. Usually in that "I want to be her" way. Mostly I'm talking about like Disney princesses, Judy Garland's Dorothy, and Ozma from Return to Oz. These were crushes I didn't realize were crushes until I was much older.
As a kid, I was raised super fundamentalist and was pretty sheltered. I figured out at some point that gay men existed. But what I didn't know until I was like 11 was that gay women also existed. I remember I had a Friends calendar (lol) and it had quotes and one of them was about a lesbian. I didn't know what a lesbian was, so I asked my mom and she told me. My first thought was, "Women can be gay, too?!?"
Once I realized that I was like, well shit. I really like women. Maybe I'm a lesbian. Around that time I was starting to finally have fuzzy tingle grown-up feelings down there and it quickly became clear that women turned me on, and they turned me on more than men did.
What really confused me is that I had strong romantic feelings for men, but sexual feelings for women. And it was exclusively like that for so many years, I didn't know what the hell my deal was. I would be completely head over heels in love with a guy, but imagining having sex with him, ever, grossed me out or at least didn't turn me on. Imagining being with a girl sexually really turned me on, but I couldn't imagine an emotional connection. There was no mental attraction to women.
I think around this time I learned that it was possible to be bisexual. Although I wasn't convinced that I was - I thought I'd eventually "settle" on one or the other.
Two things changed for me when I was a teen: Ewan McGregor in Velvet Goldmine was the first man I was able to find sexually AND romantically attractive. All the guys I'd had crushes on before this were basically scrawny, tall, and dark haired. He's average height and kind of auburn. Turns out, I'm aesthetically attracted to tall, scrawny dark haired guys, but I'm sexually attracted to average height blond or red-haired guys. So that solved that. (I rarely like the tall, scrawny dark haired guys anymore.)
The second thing that happened is I kind of fell in love with Utada Hikaru in like 2004 or so. (As much as you can fall in love with someone you've never met.) That was the first time I ever felt real, strong romantic attraction toward a woman, and not just sexual attraction.
Although to be fair, I think Milla Jovovich was my test run. I think I was in love with her when the Fifth Element was new and I just wasn't willing to admit it to myself. (I was really "afraid" of being flat out gay because my family are assholes.)
tl;dr - I was always bi, I just didn't know it. I was attracted to men in every way except sexually, and attracted to women in every way except romantically. Ewan McGregor and Utada Hikaru were the missing links for me and convinced me I was actually bi.