How can sexting be cheating? I mean, on the same assumption, even watching porn could be considered cheating
Nope. They're not compared on the same grounds. Watching porn isn't actually watching the people having sex in real life. Sexting is actually talking to someone real behind the screen because you are emotionally/physically unsatisfied by your partner. Instead of talking to them about it, you prefer finding another alternative? That's not how a relationship should work. Anyway, sorry for a bit off-topic OP.