I first got my computer after learning Ms Word and Xl then i found out about porn on the net and i had to sign up, being gay I had a ball, no pun intended, I never felt guilty looking at such sites and now spend little time doing so. However i can relate to your experiences of being introduced to sexual behaviour and feeling bad, guilty and dirty. I have fond memory's of one abuser and now realise that what he did was wrong, that he introduced me to something I wasnt mature enough to handle.
It seems that you see what you are doing as acting out and shameful. I am of an age where I can remember pin up girls in skimpy costumes as almost hardcore porn and they were in most auto repair shops every where, so in my view looking at porn as a male thing going back for many years. This would posibly not be a problem for you if you had not been abused and made to feel uncomfortable as a child, put the blame where it belongs at your abuser and the person that abused him and give yourself a break.
I'm sorry that your Mother is unable to give you what you need, I wonder if she is carrying guilt in that she wasn't able to protect you, even if she is a therapist, they have hang ups too. That being said good on you for sticking up for your validation re what happened to you.
Keep up the good work and try to remember that you are doing the best you can, or you wouldn't be posting here.