Are there many people who can honestly say they were completely open-minded, and weighed up all the "evidence" on a particular issue - e.g. capital punishment, sentencing of criminals, feminism, climate change, etc., etc. - before they formed a concrete opinion?
Do we arrive at our opinions through reading every piece of research or analytical commentary that was ever written on a particular subject? I'm not sure if that would even be possible. I think people's opinions are initially influenced by what their parents believe and then by their peer group, and these factors combine with experience to give us a sort of "template" which tends to influence how we think about other issues.
Speaking personally, I acknowledge that I have a left wing bias. However, I do try to find out in more depth about various issues and read different newspapers/watch different TV channels so that I can at least understand how people have arrived at ideas different from my own. What I find difficult to believe is that there are people who have no pre-conceived ideas and are completely "neutral" in forming an opinion on each and every issue.