The definition could be different for every relationship, but most people would consider making a personal connection with someone else, especially for sexual reasons, to be cheating.

With porn, you're not actually making any kind of connection with them, so most people don't care about that. Some do, but for the most part it's not an issue.

Now, if you were subscribing to someone's Only fans or something like that, talking to them, requesting things, then most people are probably going to find an issue with that as well.