I could only see it as being anywhere close to cheating if:
1) One partner agreed not to watch it and then watched it anyway.
2) A person watches it to the point of neglecting their partner.
Even then, I wouldn't consider it "cheating," per se. It would be neglect or a breach of trust, which are still both quite serious matters.
Personally, I like porn, and I'd be bummed if I wound up with someone who wouldn't want to watch it with me.
It's a great way to get hot quickly or to get ideas. Or you can get a good laugh, if it's really bad or cheesy.