Thank you for your question. I am sure that by asking you are also helping others that are in a similar situation. Effectively, most people will ask themselves questions about their sexual orientation at some point in their life, so you have to remember that your questioning is completely normal.
You seem to have a clear comprehension of your situation, but having trouble how to deal with the impact of living in a religious family and accepting/expressing your thoughts about women. It is a very common thing to have difficulty accepting and being sure of your sexual orientation. However you have something that not everybody have, a friend that is open and willing to help you through all of this. You might want to ask yourself what is preventing you from answering personal questions and what makes you uncomfortable. Are you afraid of your friend’s judgment? But she herself is bisexual, so it is unlikely she would judge you. It may be that it is so hard for you to acknowledge your attractions and desires that even stating it out loud to your friend may seem threatening.
With time, you may end up being more accepting of yourself –but be patient with yourself too!. Given the religious context in which you were raised, I am already impressed at how open you are and how you are able to perceive that you may be attracted to more than one gender.
On the other hand, some individuals have only mild or almost no attraction to either gender (the term used for this is « asexual »), and that is fine too. However, the fact that most of the time you agree with your bisexual friend and that you are also interested in straight and lesbian pornography suggest you might only be in the process of discovering yourself and figuring out what is attractive to you.
It doesn’t really matter if you haven’t been in a relationship before because you usually can tell when you look at someone or when you get to know them better whether you are attracted to him/her or not. Being in a relationship can confirm your doubts, but most of the answer will come from self-reflection and a deeper understanding of yourself and what exactly makes you happy. You wrote that you’ve never really felt a strong attraction to either sex, but attraction is an individual thing and also changes through time (ex: you might prefer a specific type of person when you’re youger and a couple of years later a totally different type).
Once again, the most important is to give yourself some time : don’t rush anything, as time goes by, you will grow to understand and accept yourself better. At the moment, I noticed that you are protecting yourself from the judgment of others by saying things you don’t necessarily think (ex: when your father and brother are watching porn) or just not answering questions (ex: when your friend asks for your opinion), but this is also a completely understandable stage along the way to more complete acceptance. You are not obligated to do things you’re not comfortable doing.
However, focusing your thoughts on accepting your attractions and desires rather than thinking about how others might judge you can be a first step in understanding who you are.
I hope this helped clarify your situation a bit more and feel free to write again if you have any other questions.