I'm not going to babysit so my ex-wife can mess around with other guys
Why do you care if your ex-wife is involved with other men?
My ex is the biggest bitch I know. She is totally incapable of managing her own life
It is hard to see an ex with somebody else but you cannot control them and you do have to take care of your kids. Not taking care of your kids to punish her is wrong and might turn your kids against you.
She used to cheat on me and now she's going to pay.
You are responsible for your children, and watching your own child is not "babysitting," it's parenting. Your ex is free to do what she wants, and as long as the children are safe, it's not for you to get involved.