I am new here and have been reading some posts.
My husband cheated a year ago. We are still working through a lot. Porn is one of our problems.
I do not agree with the "men are visual" theory. There's a lot of info out there to prove the opposite. We are all visual creatures, men a little more. We are taught it is acceptable for men to do this and to just laugh it off. I'm going to stop there.
I do think it is a concern for you. It doesn't matter what kind of porn he is looking at.
He is hiding it from you. That right there is a red flag. I've gone through this with my husband. Even women who are ok with their husbands looking at porn are surprised to find out how much they watch without their knowledge. Porn creates secrecy and lies.
It eventually (in my case) took over my marriage. My husband eventually preffered porn over real intimacy. It does become a form of betrayal as it escalates. Porn has become more of a problem since the internet and how readily available it is. I have read so many books on this while working on my marriage.
He's looking at photos of your friends on facebook? An even bigger red flag. That is not ok and is too close to home. Visual or not. That is crossing a line, real people, real time.....
I would definitely talk to your husband about it and tell him how you honestly feel. Hopefully he respects your feelings enough to stop. It took my husband years and we're still not through it completely.
Good luck