I'm bi, and you sound a lot like myself early on when I wasn't sure. It was the same with me, I'd watch gay porn from time to time, not really thinking anything of it. Then, I got really into the idea of doing stuff with another guy. At that point, eventually it just became obvious. I haven't lost any of my attractions, I didn't "turn gay" or anything. I just realized that I still like girls, but that I like guys too in a different sort of way though.

As for only having had the one crush, that's probably that your just not naturally a very romantic person. I rarely get crushes in that sense. I haven't had a crush in a emotional, romantic sense on anyone since I was like 13. I often become interested in other people of both genders sexually, but emotionally my mind just never really gets involved that much. I'm sure it would if I were with someone and cared about them, but I haven't really experienced that very much. There's a good chance your probably similar in that sense, since it sounds like you don't really typically have emotional feelings for girls or guys.

I'm not entirely sure about the whole "born this way" thing is true of everyone. For some people, like myself and likely you too, it kind of evolves over time. It may have always been there on some level, but as time goes on things just change. It becomes more obvious to the person for various reasons.