What expense does a person suffer if they are being cheated on? Some forms of cheating involve financial irresponsibility, but not all. Some forms involve health risks, but not all. So let's not generalize all cheaters as perpetrators of harm to their first partners.

Judge all you want, but if it ain't illegal don't shove your values down people's throat.

Do you realise you're using the word "cheater"?

Having multiple relationships, known to all parties is subjectively wrong and we can argue about it as much as we want.

Agreeing to a monogamistic relationship and secretly breaking that agreement, as implied by your use of the word "cheater", is objectively wrong.