It’s always scary and confusing when we find out something about our parents that we didn’t expect. We need to remember, however, that each of us has private thoughts we would never share with anyone else. Each of us has dreams, sometimes, that shock us. Or we have sexual fantasies that excite us, but we would never want to repeat in real life. For some people, even happily married people, looking at pornography together adds a certain oomph to their sex life, especially among older men who may need more stimulation. One of the problems with pornography, however, is that it often demeans women, and photographs them in positions that men themselves would not want to be photographed in. Also, it keeps upping the level of excitement necessary for pleasure, that would be difficult to replicate in real life. The fact that your dad does it in private probably means that he has reservations about it too. I would be more concerned with his contacting dating websites. If this continues to haunt you, I would not approach your mom. I would speak to your dad in private, simply by telling him how you came upon the information, that it frightens you because you wonder about the marriage, and ask his reassurance that things are OK between him and your mom. You are not asking him to stop, you are not preaching at him, you are simply voicing your concerns about the stability of your family, but beyond that, whatever he does in private is really his business.