What about kissing? Sexting? Secret messages? Active online flirting? A dating app “just to look”? An emotional bond someone deliberately hides from their partner?
This is where the word infidelity stops meaning the same thing to everyone.
One couple is comfortable with porn and flirting. Another considers erotic messaging more serious than a spontaneous kiss. For some, physical sex is the absolute boundary; for others, hidden emotional intimacy feels more threatening.
The riskiest situation may be when two people have never actually discussed the rules because both assume “surely this is obvious.”
So I’m less interested in abstract morality here than in just how differently people draw this line.