I've been thinking about something that's confused me lately... I started my 7th grade year, at the age of 12, thinking I was completely straight. I had a crush on a red-headed girl who flirted with me. Nothing happened with her, but I was turned on enough to rush home and jerk off after a couple flirtatious gestures. But then mid-way through the school year, I got a crush on a boy in my science class. I didn't realize it was a crush at the time, but I was fascinated with him, wished I could be his friend, and had fantasies of rubbing up against him. By the end of that school year, at age 13, I walked all the way home on the last day of school. I was worried that if I kept thinking of guys, I would "turn out gay." (Which, of course, I did.) I tried to force myself to stop looking at (and fantasizing about) guys. I lasted about 10 days before I gave up. To this day, I wonder what happened during my 7th grade year? I was straight in the fall, but gay just nine months later. While I've read that puberty and raging hormones can cause fluid attractions, I'm still baffled as to what happened...
A certain amount of fluidity in sexual orientation can happen, but there are other factors too. Boys are socialized to find girls sexually attractive and vice versa. It's quite easy for people to think what they feel towards someone is sexual attraction when it's actually something more like aesthetic appreciation. Add to that that as a teenager it's especially easy to get aroused and it's fairly easy to get off to just about anything at all (as Xander says in Buffy the Vampire Slayer, "Linoleum turns me on.")
