This is always a fun convo!
I think it was gradual, and I knew way before I knew to label it as bi/pan (I like the bi colors more ๐ ). Boys even as early in elementary school would sometimes talk about how pretty or hot their teachers were, and I didn't realize they usually reserved such comments for the female teachers. So I started think it about any I found attractive, men included.
Past that, closer to teen years, I'd have fantasies of being caught masturbating, but the subject of those dreams also weren't restricted to just female figures in my life. Again, I didn't have a label for it, but it never felt weird or bad.
Later on in mid-high school, I was driving my friend home from a party and he asked me if I'd ever had a girl suck me off. I hadn't, and answered accordingly. He asked if I wanted to know how it felt and started massaging himself over his shorts. Horny vibes took over and I agreed and that was that. I think I tried to deny it a little, but I was always in the LGBTQ+ crowd in HS anyhow and it helped me accept it a lot quicker. The guy turned out to be gay, and I think people are fucking hot, regardless of gender presentation or biological equipment.