The punishment for being unfaithful is perminant seperation. When a cheater realizes that he has loss the one person he took advantage of and hurt. Also, if deciding to stay with the person who cheated, punishment is not trusting him and constantly questioning his whereabouts.
Although some people like myself use to say, once a cheater always a cheater, it means he/she is easily tempted and has it within them to stray. I do know of cases where the person was unfaithful, but changed to avoid all the drama that came with infidelity. It is definately possible to learn a lesson and not stray, but that comes with life experience and maturity.
1. There isn't exactly a punishment, like, it's not back in the day when you cheat you have to walk around with a scarlet letter or made to leave town for committing adultery. Although, you can say a punishment would be, hurting someone who loves you, taking away the trust and now having to be under a microscope. Losing someone who loved you, losing your family, if there are children involved. Divorce. Losing someone who was your best friend. Losing respect from the person who once trusted you.
2. I don't think once a cheater always a cheater. Some people cheat because they are immature and they grow up and learn to value relationships. Some people cheat and feel regret and remorse and realize the consequences to their actions and don't take chances like that again. There are a million reasons why someone may cheat. Perhaps they were involved with someone who wasn't the right person and found it easy to cheat and when they met the right one, they didn't feel the need or desire to look elsewhere.
People learn and grow from experience. Some do and some don't, I think it varies amoung individuals.