I think there are many different reasons why. But the main reason is you’re a awful person (if you’re the cheater.)
If it weren’t for STDs, it would be slightly less unethical.
Relationships are built on trust. IMO, once you step out, you have violated that trust and the relationship irreparably.
If the person who you’re cheating on loves you, and you know this, you’re an especially terrible person.
I think being cheated on is comparable to rape. When you find out, it’s crushing,and infuriating. You feel violated. It’s SO simple. Don’t have sex with other people when in a monogamous relationship.
If you cheat on people who love you,and maybe you used to love, you deserve to be alone, and miserable. And die that way.
Just break up . Then you can FTW.