Its not good to see things as "black and white." Obviously, some things are outright bad and should be avoided.
It's all about moderation. If you're watching porn, but youre also able to have sexual relations with a woman, then its no problem. If you're sitting home and watching porn 24/7 and you lose your wife/girlfriend, lose your friends, lose your job, and blow all of your income on porn, then you have a problem. If watching porn is something you just happen to do, within the context of other things, its not a problem.
As you stated, it becomes a problem when you watch so much porn that you cannot keep an erection during sex.