Took years for me. I was indeed attracted to guys and girls at the same time throughout much of middle school and high school. Attraction waned to just guys in college (probably by 20 or 21.)I wouldn't say it's common, but I've definitely come across elements of biphobic feeling in the community. Not necessarily outright animosity that often, but the peddling of old tropes like the idea that bi people are just greedy or they're too scared to admit being gay, which generally pisses off bi/pan people. The latter is annoyingly prevalent because I think the 'oh i'm not gay, i'm jut bi' phase is quite common when people are coming to terms with their sexuality and people often assume that all bi people are still stuck on that stage. I know I had a moment where I thought I was bi. Admittedly it lasted all of about a day before I realised that I was strictly dickly, but the point still stands.
It was a transitional phase, but the attractions were very real and took me a long time to come to terms with. Things have evened out a lot since then.