I’m in a married, committed relationship with my partner because that is how we both decided this is the kind of relationship we want.

We also have friends who are married but see other people openly, and both parties are aware, and their additional partners know that they are married.

Neither one is right or wrong, it’s what works for them.

In this case, both people went into their marriage with the intent to remain monogamous. These two did not honor that. That is why is wrong - it’s not about seeing other people if that is the initial agreement. It’s the lie, which both of them did.