With regard to the possibility of having been sexually abused: my reaction to the thread is that age five seems unusually early to begin to masturbate. To me, masturbation at that age is consistent with there having been sexual abuse.

With regard to the theory that overexposure to images of abusive sex: I think a relevant question would be: What brought you to those images and kept you returning to them?

Finally, sex is a form of intimacy and therefore attitudes towards sex will be influenced by attitudes towards intimacy in general. How willing are you to share your personal thoughts in real life with appropriate other people around you? How much closeness do you allow to others in your life? If thoughts of allowing closeness to others sounds horrifying to you, then it is quite possible that those attitudes would spill over into an aversion to sex (whether or not there was sexual abuse in your past).

I agree with others who suggest that speaking with a therapist who is well-versed and competent in sexual issues could be very helpful.