For me it changed a lot over the years. I started getting attracted to guys pretty soon after I started getting attracted to girls, around 12 IIRC. But back then I never wanted to date a guy or have any relationship with one. I only wanted relationships with girls and I was only curious about sex with guys. I had a few girlfriends from 14-15, one of which I lost my virginity to, and most ended in what could nicely be called apocalyptic disaster. After that I didn't wanna date anyone for a while but I started getting really turned on by the thought of finally having sex with a guy. I even joined one of those "meet up for sex" websites when I was 18 but I was too scared to ever go through with it, lol. Somewhere in all that I decided I wanted to try maybe dating a guy, something I never thought about at all till then. When I was 20 I met my first/current boyfriend and we got together and it was a bit weird at first getting used to dating someone who could lift me up instead of the other way around but now I don't think I could ever date a girl again. I find guys way more attractive than girls but I still get turned on by straight porn and every once in a while kinda have a short lived urge to have sex with a girl again. But mostly it's all about the guys now, lol.
I have no idea why things changed so much. I don't know if things like that are common or if I'm just weird, but that's the saga of my bisexuality. I think I'm more of the first type than the second though. When I'm looking for gay porn I usually like the ones with kinda feminine looking guys but I'd much rather have sex with a guy who's more on the manly side, so I'm not really looking for a "chick with a dick" type of thing. It's kinda weird though, because with my boyfriend I get really turned on by being the more submissive one in bed, but with a girl it's the opposite. Maybe I'm misogynistic-sexual or something but a dominant woman turns me off as much as a dominant guy turns me on. Like I said, weird, lol.
Awesome topic though