Most people understand that sex with others is prohibited in a monogamous relationship. But modern relationships also exist digitally: someone can flirt for months, exchange intimate photos, maintain a highly emotional bond or use dating apps without ever meeting the other person physically.
That is where “but we never actually had sex” becomes a weak argument if the partner understood that behaviour to be outside the couple’s agreement.
This does not mean there is one correct boundary that should be assigned to every couple. Some permit flirting. Some explicitly agree to sexual contact with other people. Some consider private messages personal territory.
The problem is not that one couple’s rules differ from another couple’s. It is when two different versions of the contract exist inside the same relationship and neither person realises it.
That is why conversations about fidelity are useful before infidelity happens — when nothing is wrong yet and there seems to be nothing to discuss.