Is watching porn cheating?

I think it depends on your relationship. If you are allowed to "look-but-don't-touch" then it shouldn't be cheating. If you are constantly questioned and have to explain that the person who texted you is a family member/ co-worker, then your partner would probably consider it cheating. Really, you should just ask something like "Hey, I heard that some people consider porn cheating. I'm curious about your view."