Is it bad to go back to someone after they've cheated on you?

Not necessary. We all make mistakes , if u love each other and willing to forgive and forget than good luck.
Maybe. Forgiveness, if one is able to, might help you reconcile. It has to be given with the clear understanding that saying " i'm sorry" is not enough. Truly sorry means also that the offending person is willing to commit to not doing it again and do everything possible to make it right. Saying " i forgive you" also means you are willing to put the offense aside and not use it as a weapon in any way.