Well, clearly when your partner becomes affected by your watching porn, that's when it crosses the line. Watching porn is not wrong, even if you are in a relationship. Who is to judge? I like to look at pornography (pictures, not movies) when it is more artistic and has more qualities than just the kind that get you to masturbate at it.

I also think people SHOULD be confronted by the thoughts of dating other people while they are in a relationship. I think you should be confronted by thoughts about other women, and then being glad to turn them down because of love. If you have to deny and force yourself not to think about those things, how strong is your love?