I don't understand why cheating is bad
What makes you think cheating is okay?
I don't think it's good or bad, but I've been cheated on before and that hurt a lot.
Since you have experienced the hurt caused by cheating, it's expected that you would understand why it's wrong to cause other people  to feel that way. Cheating is bad behavior and it should not be condoned. Instead of cheating, a person should work on improving their relationship or simply end the relationship if they are not happy.
I think it's probably OK though.
It's not OK. Cheating is really immoral and it means you don't care for your partner. It's shameful, really.