I've continued to wonder how it took me so darned long to figure out I'm bi, and while reading a book on bisexuality this evening a possible answer occurred to me. Or maybe I should say, part of a possible answer occurred to me.
Reminder: I questioned my orientation some when I was 19, returned to lying to myself and insisting "I'm straight!!!", then when I was almost 23 I finally came stomping out of the closet and was upset with myself for not knowing it sooner.
Ok, so I guess I wasn't stomping out of the closet. After all, it took me about six months to tell my boyfriend. Don't ask me why, I really don't know. Probably because I was uncomfortable with myself at first, and then didn't see why it should be a big deal.
But back to why and how I fought being bisexual for so long, and lied to myself that I was straight. I think it basically comes down to two assumptions I had.
1) To begin with I believed that everyone was straight or gay. You liked guys or girls. I obviously liked guys, as was the norm, so I couldn't possibly be into girls.
Looking at it now, that was some pretty narrow thinking for me. Then again, it's what a lot of people assume. Plus, I was so busy struggling with ADHD and a speech disorder (I couldn't even say my own name correctly!) that I may have subconsciously tried to avoid being different in any additional way when I hit puberty.
2) I thought that everyone just knew their sexual orientation up front. I couldn't understand why people would have to spend time questioning it, or how they could even change their labels.
This second fact means that even once I learned about (and finally believed in) the existence of bisexuality, I was unable to apply the new found knowledge to myself despite my obvious attraction to my own gender. Heck, I even remember thinking things like "Why am I getting nervous around this beautiful girl like I do around cute guys? It's the same sort of feelings, but that's impossible. I'm straight!" That I wasn't should have been obvious, but I couldn't even consider it.
As you may have guessed from the last paragraph, I did deny the existence of bisexuality when I first learned about it. I don't remember who told me about it or when, but I thought it was a joke. Probably because it wasn't someone coming out to me, but rather someone telling me about this weird (to me) thing. I'm glad I can say, though, that I didn't doubt it when a friend came out to me as pansexual. Was I stunned? Yes. Did I argue? No. I took it for granted that someone would know their own sexual orientation.
...and while maybe not the whole of the answer to my original question, that last little bit is probably the biggest reason why it took me so long to come out to myself.