It has a lot to do with why you are watching someone naked.
Pornography is the graphic presentation of the sexual act, geared towards self-gratification. It is always selfish, because it removes “the other” (your spouse) from the equation, and changes the sexual act into one that is entirely self-centered, rather than the giving act it is intended to be.
If you’re watching someone naked, it could be in a movie (an unfortunately prevalent occurrence nowadays) in which case, assuming you weren’t seeking it out, and don’t dwell on it for sexual gratification, there’s nothing wrong with watching it (to my knowledge) After all, we are not ashamed of the human body, but rather believe that it is to be treated with respect, and only revealed in full to your spouse.
If, on the other hand, you’re watching someone who is nude, but is nude in such as way as to be be sexually explicit (there are a lot of these types of sites, claiming to be artistic, but are basically just a fancier version of playboy, where the women are put up to be stared at and used for gratification) then it’s very wrong, because it is again rejecting the unitive aspect of human sexuality.
Nudity is not inherently wrong or sinful; nudity intended to elicit a sexual response outside of the confines of marriage, is.
(An example of this distinction would be: as an art student, I study the human form so as to be better able to draw it. To do this, I employ nude models. I am not viewing them for sexual gratification, I am viewing them to learn. This is not sinful. If, however, I start to fantasize about my nude model, I cross the line into sin.)
You’ve been making a lot of posts like this. I really suggest you read the Theology of the Body, it’d probably clear up a lot of your issues.